Well...
Here goes sort of an "industry secret", hardly a secret, but just not *that* known (yet?), and it's, by very very far, the best audio enhancing tool I've ever used... weird I rarely hear anything about it...
http://www.crysonic.com/spectralive.html
It does not add "color", it keeps the signal's phase with no distortion, it add's kinda like what you add to your vision when you place yellow lenses in front of them: clarity and warmth. You can over-do-it, but try to look at it as sugar, salt and spice...
...just that bit more and it's ruined, that bit less and it could be better...
IMHO, it's a bargain and these guys don't know what they have... but you can test the demo and see if it's the kind of "warmth" you're looking for - it does not distort, it's "transparent" warmth, if one could call it that.
Ozone 3 is (finally) transparent, totally and period: just don't use the exciters... the analog EQ modelling is the finest of finest, if you count off Waves EQ's. I don't like the compressers that much but I love the inteligent loudness maximizer (which I don't use) - it won't warm up (or cold down) the sound though - but we're talking warmth here, so forget this.
IK's T-Racks24 mastering package is very warm, but stops at 48 khz, it's not as intuitive as SpectraLive and induces typical transistor analog phase distortion, which you may want, and should add only once, in all at the same time, preferably, not to have phase problems, and it's a mastering tool like Ozone, SpectraLive is just an effect.
All "tube modelling" most VSTs are too... er... "what can I do with this?" IMHO... But I happen to like (love) BlueTubes, don't really know why, they sound... cool (authentic, transparent and as sweet and round as the real things... but that's not analog warmth, it's tube warmth - clean and sweet, not grainy and fat - try Steinberg's Magneto for the later, a well saturated tape maybe your kind of warmth - not transistor, nor tube).
(GeeJee - here you go, you wanted info on warming up, here's above the main thread, follow up

)
Kind Regards,
AMR
http://www.alvaromrocha.com