BPP - are you mainly using the UAD on loud sources / stuff with a lot of high transients, and using the mild compression / saturation effect that tape brings to the party? Or is the process noticeable on more easy going sources too?
And is it more of a culmative effect, i.e. stick it on 4 tracks and you're not really going to get the benefit, but stick it on all 24 tracks of a mix and then you're cookin'?
You can hear the effect if you put it on the stereo buss only, but, to my mind, it works best when treating the tracks, as a cumulative effect.
Yes, the difference is subtle, but as you say, cumulatively, it's quite apparent.
The amount of harmonic distortion I get is dependant on how hard I push the input. Using it "clean" still adds tape character.
In answer to your other question, I have used the A800 on vocals/piano/drum buss/guitars (acc & elec)/and orchestral tracks. Most of the tracks peaked around -9db.
There are plenty of good presets provided, including an excellent 60's style lo-fi option. (really does sound like authentic tape distortion - think Duffy, that sort of thing)