[Timo]

Feature sets, to my knowledge:

• Virus A has 12 voice polyphony, 2 main oscillators + 1 sub, 2 LFOs, one FM mode, Chorus, RingModulation, Distortion effects, but no Phaser effect, 3 sources and 6 destinations for modulation matrix, 16-part multitimbral (including aux-bus & audio output channels), and 20-Bit D/A.
• Virus B/Indigo/kB =, same as Virus A but with 24 voice polyphony, up to 3 main oscillators + 1 sub, 3 LFOs, 5 FM modes, additional Phaser effect, surround sound capability, PureTuning (microtuning), and 24-Bit D/A.
• Virus Classic - Identical to Virus B.
• Virus C/Indigo II/kC = same as Virus B but with 32 voice polyphony, MiniMoog analog-emulated filter (selectable from 1-to-4 poles), 6 sources and 9 destinations for modulation matrix, 3-band EQ with parametric mid per part.
• Virus TI/Polar/TI-KB, same as Virus C but with up to 80 voices polyphony, Total Integration, additional HyperSaw, Wavetable, Graintable, and Formant oscillators, independent reverb and delay, programmable arp', 6 sources each with 3 destinations for the modulation matrix (18 destinations total), refined Oscillator Sync', tap tempo, knob-quantise, PureSemitones, USB (16Bit/44.1KHz) and S/PDIF I/O, 6 balanced (+4dB) 1/4" TRS outputs, 24-bit analogue inputs and outputs, 512 RAM & 2048 ROM patch storage, Atomizer [pending OS v2.7], 4x stereo outs via USB [pending OS v2.7].
• Virus TI Snow = A cut-down Virus TI. Identical sound engine but only up to 50 voice poly, 4-part multitimbral, 512/512 RAM/ROM storage, and only 2 analog audio inputs and outputs.

I bought a second hand Virus Indigo (B) for £400 about four or five years ago. It's a fantastic little synth, and the only bit I really "miss" is the MoogFilter of the Virus C and perhaps the Hypersaw of the TI.

The additional features of the TI are now substantial though, after all of the software upgrades it's been given. It's a very different synth to the Virus C. Much moreso than the Virus C is to the Virus B, for example.

I'm waiting until a TI mk2 arrives (I guess some time in 2009) before finally trading in my Indigo. Otherwise a TI (new or second hand) would be good if you could afford it. It would give a lot of mileage.