Additive synthesis has always been the poor relation to subtractive. The biggest problem is the number of partials needed to generate decent spectra, and their control. With up to 512 partials in Cube its plain you cant just set a separate UI ADSR for each!
Cube presents the additive engine via a UI that allows morphing between four sources (A..D). Each source can be edited for partial level, attack, decay, filter, pan and noise. Complex spectral movement is accomplished by morphing between the different sources. There are a set of preset sources (vocal style vowels, Saw/Triangle etc) for you to start from.
The UI is well laid out and presents clear information on its settings. The X/Y morph envelopes can be viewd both as single time vs level envelopes and a 2D vector on the central UI area. Some improvement could be made here as it sometimes hard to find when 'end' of the vector display is which, all envelope points are drawn in the same colour apart from the currently selected. Perhaps the end and start points could also be coloured differently?
Cube seems best suited to sounds that have movement in their high frequency spectrum. This makes it a very good companion to other subtractive synths. Electric Pianos, Pads, Choirs and rythmic pulses etc seem to be it forte. Its quite thristy on CPU power if you just ramp up the number of partials for no reason, but with some tweaking it can be made no more resource hungry than many other VSTi.
The manual is brief but well written and easy to follow. Perhaps a background chapter on additive synthesis would make it better for first time users.
So far Cube has been very stable. No crashes, just a few times the 'randomize' feature has produced a patch thats so complex it overloads my 1Ghz PIII CPU.
In summary, I'm very happy with Cube. It opens a door to a whole new range of sounds and is great fun to use.
PS. The 1.01 version was released just after I posted this review. It now animates the GUI to show the morphing process in realtime. So forget that comment about the vector display being confusing. Its a breeze to work out now. Well done VirSyn.