In my opinion, this is one of the best EQ's around. Not only does it sound great and have a great GUI, it's also very versatile and provides a number of EQ styles.
One should really have a look at the manual, it's very easy reading with nice clear graphs and straightforward plain-English writing.
On the left side of the GUI, there are 2 style knobs, one for the bells, the other for the shelves, with 3 settings each.
With setting no.1 this EQ does the very broad bells of classic EQs,.
Setting no.2 gives you a gain-Q coupled design, like that of the API 560 and SPL ranger series.
Style setting 3 gives you the uncoupled design which is more modern.
In other words the the gain-q coupled bell decreases the Q as you boost while the other gives you a true-Q response. One is more "musical" and the other surgical.
You cannot however choose these responses per-band, only per-instance. Does it break workflow? I think not, for me I do different stages of EQ, surgical to remove unwanted frequencies, musical to get a sound I'm interested in and broad to give presence. So this would require 3 instances of this EQ, not good for Cubase users perhaps, but thankfully It's light on CPU.
Cutting is also flexible, there are switches below the aforementioned knobs that deal with symmetry. You can have either a symmetrical response or an asymmetrical response. With a symmetrical response your boosting and attenuating (cutting) will be equal.
The normal response of most EQ's is asymmetrical, that is that cuts are narrower than boosts by default, and the Q's narrows the more you attenuate. Often engineers advise that for a more natural or transparent sound boost broad and cut narrow.
You can have the opposite if you wish, with a symmetrical response your boosts and cuts can have equal bandwidths.
What is interesting to me, the manual here states that the symmetry of each bell type differs, based on their extensive research on what sounds the most pleasing.
The Shelves are very interesting to me. One of the great features of this EQ is the adjustable slope on the shelves, allowing you to change their response dramatically.
Again we have 3 types:
Type one being the most common employed in the classic Eqs which creates an increasing dip at the shelf transition frequency.
Type two reminds me of my Portico Eq, it creates a dip like type one, but it also has a resonant peak around the corner frequency. This allows you to make a general boost to a shelve while at the same time emphasizing a specific frequency. So lets say we are boosting a bass group with kick and bassline, we can boost the entire bass range as we pleased while placing a special emphasis on the kick put putting the resonant peak at the right spot.
Type three has no dip and has a resonant peak.
The symmetry control is great here. Remember that little dip introduced by type one and two? Well at this point when cutting a shelf those dips will become peaks, this is what steinberg's console EQ does, here though we are able to flip a switch and have those dips become further cuts in the shelf instead of becoming peaks.
It's easier if you just look at the graphs in the manual to explain this.
Aside from all this, there are hi-pass and low-pass filters with a steepness up to 24dB. I sometimes prefer more drastic slopes, but 24dB is fine for most purposes.
You cannot have any band used for any frequency as each band is limited to a certain area. Can be annoying sometimes, but on the other hand it helps guide you.
The GUI is beautiful in my opinion, Sonalksis have the nicest GUI's around. Very professional and a lovely colour. However time has gone on since the release of this EQ and I believe it's time it was made larger (if possible) to take advantage of larger screens with high pixel count, as I do find it a little small, that said some larger GUI's are not nice to work with as they take up so much screen space.
The menu is very helpful and allows for a lot of customization. You can deactivate the graph but you cannot switch the GUI to a knobs only view like you could with Equilibrium for example. Shame, as it's nice to have nothing but the knobs to look at sometimes so you're forced to use your ears more. Sometimes a graph is just too distracting. Fortunately you can turn off the response curves on the graph (which I believe also has 3 settings) in which can you will just be left with a blank graph under the controls.
The Analyser is great and can be switch off if needed. The scale can be toggled between two settings for mix or mastering uses, and there are many more little changes that can be made in the menu. Often it's all these little things that make a difference.
The product page on KVR still shows the old price of this EQ, $259 or so, it's actually $110 now which means it's a great price. Is it really a relevant option now that we have Mr Gamble making new EQ's which much larger GUI's and many options? To me yes. I have Equality, and I still prefer Sonalksis, even with the smaller GUI. Partly because I am used to it, I know everything I am doing on it and know a lot of what I can do (although amazingly even after having it for well over a year now I am still discovering all the subtleties of the bell and shelf types) but partly because I just find it nicer to use than Equality.
Of course I cannot offer anything like the amazing options of Equilibrium, but that's costs almost $200 more.
It might sound strange, but in my mind this is the best EQ plug-in I've ever used, even when put up against Equality, Pro-Q, Digital V_2, Portico & others. It's not that it beats all these in every area, but it beats them all for me because it's so well rounded, it has the perfect balance of features and ease of use, wrapped in a very pleasant GUI with easy CPU usage and no stability problems.
My next general go to EQ is in-fact Brainworx V2 as it offers the M/S modes that Sonalksis-517Mk2 does not.
Then again I don't know if I'd like to be without Equality for the mastering stage, where it's perfect for the little changes in the M/S domain.
I think Equilibrium is perhaps the ultimate all rounder, but what takes second place for me is Sonalksis. If they made an option for the GUI to be larger then I think I'd give this a 10.
However If you find more complex EQ's with large GUI's to be a bit overwhelming and at the same time don't find the simpler EQ's such as PSP console EQ's, DDMF 6614 and Kaussa's new offerings to be versatile enough I think could be the one for you.
Be aware however of Sonalksis second hand NFR policy. If you buy this from someone you cannot sell it on, which on the other hand means if you do find this for sale (I don't often see it come up) it's likely going to be silly cheap.
Objectively I give it a 9. But it is my go to EQ and it's also my favourite overall.