Regarding the Since click. IMO, it's in fact a psychoacoustic phenomenon because the sound starts suddenly if the amp envelope has a very short attack time, and can be compensated for with adjusting the attack.
Important factors to get a pure sine without click:
- Disable all distortion stages of the Virus, including the Character settings which are often forgotten. Set Analog Boost Intensity to 0.
- Set Punch to 0
- Set Phase Init to 1or 127 ( OSC 1 and OSC 2 in Phase) or 63 (OSC1 and OSC2 out of phase). If you're just using OSC 1, Phase Init can be any value larger than 0, as Phase Init will start OSC1 always at 0 of the wave. See Virus TI reference manual on Phase Init.
To compensate for the perceived click, the Amp Envelope Attack can be adjusted using the Attack knob. Additionally it can be refined, by using a Matrix slot with the 10% constant to fine adjust Amp Env Attack.
You can also use recursive envelope modulation to change the curve of the Attack phase by using a Matrix slot where the Amp Envelope is the source to modulate the Amp Env Attack. To me, this gives the best result for a fast smooth attack without perceived click.