Just some quick consideration on the reasons about this small analysis I performed.
First of all, it hasn't to be considered an exhaustive and complete analysis. My intention was only to perform a quick check, to verify if at least the frequency response of the simulated amp model, that is the simplest part to model, was right. From my point of view, if the amp sim is not able to mimic not even the frequency response of the original, there are few chances that it will reproduce the more complex aspects of the original amp accurately, for example the dynamic behaviour of the latter.
This doesn't mean that such an amp model can't sound good on its own, instead, you could like it even better than the original! Nevertheless, its modeling can not be defined accurate. It can only be considered a a good sounding amp on its own, but, despite marketing claims, not an accurate model of this or that amp, sorry....
I've to add that I've also checked for accuarcy in modeling the dynamic behaviour of these amp sims, but I've not posted any screenshot to avoid overcomplicating an already complicated topic...
It has not been a super-accurate check in testing THD in LTSPice versus all the amp sims, because such a test, would require accurate matching on the signal level feeding the amps, and this is a complicated matter when you are comparing a "real" simulation in LTSpice, where you can use "real" signal voltage, versus an amp sim that lives in the VST world, where the levels are confined in a -1 to +1 range...you know...
Nevertheless I exploited a characteristic of the JCM800 to conduct a "reasonably significative" analysis: the JCM800 circuit exhibit a "shift" its frequency response when subjected to low input signals levels versus high signals levels. In other words, I've simulated the JCM800 circuit in LTSPice, stimulating it with noise at 5-6 different voltage levels, from really low to really high. Did the same with amp sims, and looked for "similarity" in the frequency response "shift".
With this considerations in mind, I'll give my point of view on the amp modeler I've tested, both in frequency response accuracy terms (more objective evaluation) and dynamic behaviour terms (more rough, semi-scientific evaluation):
I'm a bit puzzled by the Amplitube modeling of the Twin Reverb 65, especially beacuse that is an "Officially approved by Fender" specimen. Frequency response seems to be really off.
Their generic "British tube lead 1", not officially approved by Marshall, seems instead, paradoxically a lot more accurate in modeling the behaviour of a JCM800. Dynamic behaviour seems accurate too. Good!
To the people that were suggesting that the Twin Reverb 65 is part of their "old" Fender1 collection, and so that it would have been better to investigate the "new" Fender2 collection, I reply saying that, unfortunately, in this second collection, they did not model a Twin Reverb65; They focalized more on the "tweed" models. Probably they think their first collection already did a good enough job in emulating Fender amps they modeled at that time. Who knows....
They seems to model accurately all the amps I've tested, included the nuances that leads to a frequency response alteration on certain amp models, for example, when having the volume pot at really low or really high values. Dynamic behaviour it's good too. Really good Job!
Despite not being as meticulous as Overloud TH3 people in modeling "frequency response volume pot nuances", they did a discrete to very good job (depending on the specific amp model) in modeling the amps I've tested, both, in frequency response and dynamic behaviour. Considering the price of this unit, this is a really really really good sounding solution, except for their cab simulation "system", that completely sucks.
For example, their Twin Reverb 65 2x12 cab, has a frequency response that have some peaks and deeps, like real guitar cabs must have, but problem is that it extends "almost flat" up to 20Khz: no guitar speaker behaves this way, that's sure.
I've discovered, however, that it is this way, because, when you connect headphones to the unit (and behave this way also when you set the unit in what they call "mixer mode", as opposed to the "amp mode") they "superimpose" a strange low pass filter to the the cab sim selected by the user, and their conjuncted work act to "resemble", finally, the frequency response of a guitar speaker...well...a bad one, I mean...
In other words, try the Digitech RP360 with external guitar speakers impulses and this unit simply SHINES! Try it using its internal guitar cab simulation system and you'll fall in a spiky mosquito fuzz world.
An alternative solution, is to use the equalizer included in its internal effects modules and cut the hell around 4Khz (8-12dB), boost 4-5 dB at 125Hz, cut a couple of dB at 8Khz. It works more or less, from good to really good, with all the cab simulations included in the unit.
PEAVEY REVALVER 4:
Similarly to Overloud TH3, they have really accurate modeling going on here. The possibility of tweaking the amp sims circuits in a "SPICE like accurate way" it's simply mind-blowing!!
They also seems to have paid particular attention to the power amp-speaker interaction: It's more than a suspect that this is the reason for that "particular" peak in the really low region, around 50 Hz, for the TwinReverb65 emulation, and the "light" boost in the high freq region in comparsiono to the LTSpice simulation.
Revalver 4 has in fact a setting for choosing the power amp-speaker interaction, where the user can choose between 4 possible "kind of interaction". Moreover there is also the possibility to tweak the "fspeaker interaction" parameter, read the amount of interaction between the speaker and the power amp.
In tube power amps,in fact, being the latter naturally functioning as quasi-current drivers, their interation with guitar speakers (that have a peak impendance in the region between 40-100Hz, usually, and a rising impedance starting from 300-400 Hz) cause a "boost" at "low" freq and "high" freq.
Considering that for my simulation in LTSpice I've used as load for the output transformer a 4 Ohm resistor, hence not a guitar speaker, I should have set this "fspeaker interaction" parameter to the value 0: I've discovered this after I had already posted the screenshots, sorry.
Nevertheless, you can see that the power amp-speaker interaction, can't cause such big differences in frequency response as the ones visible for the Amplitube case. This, in conjunction with my previous LTSpice graph, should also turn away the suspects that the user @Dave Clark had about this point.
Hence, a part from that peak caused by my fault, Revalver 4 results seems to be spot on with LTSpice results. Outstanding.
GUITAR RIG 5:
It seems to be sufficiently accurate in terms of frequency response. However, on the TwinReverb 65 analysys, it seems to deviate from the LTSpice results when analyzing the Twin Reverb 65 with the switch "bright" active (this analysis is not shown in my screenshots for none of the amp sims, nevertheless I've tested also that
JCM 800 seems sufficiently well emulated.
LG2AMP (from exe consulting, read the user @Dave Clark):
This is not intented to emulate any specific amp, at least from my knowledge, so there is no sense in evaluating its accuracy in emulating a TwinReverb65.
It has to be intented, as from my understanding, as a general purpose clean amp.
Nevertheless, if it's supposed to emulate a generic amp, in the meaning of a generic TUBE guitar amp, judging from its frequency response in the high frequency, I'm humbly dubious on how (and if) the effects of grid stoppers resistors/non-zero impedance source stage feeding another following stage/miller capacitance have been modeled: in all the guitar amps, emulating such aspects of the real tubes, should lead to a subtle (or less subtle, in function of the circuit topology) hi-freq attenuation. Here I can see no such effect.
Still, this is a really good sounding amp sim! Bravo!