Due to the number of routes one might take in designing a sound with the Virus, I don't think there's one particular setting that can be adjusted to make all Virus patches sound sizzling/warm.
I do find it a bit surprising to hear it said that the Virus (and for that matter, Sylenth and z3ta) sound mediocre; they all sound fine to me.
Edit: note that the Nexus I'm referring to is the reFX Nexus 2... if you were talking about the Tyrell Nexus 6 or some other synth, just ignore everything I said, though please elaborate on what Nexus you were referring to and provide a link to some demos or something in that case.
So far as the Nexus sound is concerned, I listened to their demo video and can't say I heard anything I thought was new or original. I'm sure the presets are useful if you're looking to imitate the current popular sound though, as it does have a very polished sound to it, no rough edges. Keep in mind that the Nexus seems to layer up to four sounds per patch, so it may be a bit unfair to compare a single Virus patch with a Nexus patch.
I didn't look much into how programmable the Nexus is, but considering that the video didn't focus much on that, and given it is a ROM based synth, I would not expect much on that end. I would be surprised if it had anywhere near the capabilities as the Virus has in that regard, but given you seem to be concerned with presets, I doubt that's a concern for you.
In the end, it all comes down to what you are looking for. The Nexus is probably good if you're looking to throw a song together in a hurry, while the Virus would probably make the Nexus look very bad if you decided to create some new sounds (based my my own experience with ROM-based synths, though I admit I have not used the Nexus, and for all I know it is incredible to program). Also, in my opinion, at least, there are plenty of good sounding presets on the Virus, too. The Nexus definitely wins on price though, gotta give it that.
Sorry I couldn't help you make your Virus sound how you like; maybe somebody else will be more helpful in that regard. Probably flabberbob, that guy is like a sound design god or something, judging by how he answers seemingly every question about sound design on these boards.