>How do THD distortion figures equate to a particular tonal quality >and how does that tonal quality influence the concept of "neutral" >sound.
Even order distortion makes the tone fatter and richer than a neutral tone...the instruments sounds fuller and ewarmer...guitarists love this, and so do listeners like me.
Odd order distortion makes the music sound thin and a little garbled.

>Since most recordings made in a studio have some kind of processing >installed, are they neccessarily nuetral?
You are right. Hence when comparing amps, we should use the same recording, to remove the effects of the recording. If it sounds richer & fuller on one amp versus another, it's the amp.

>SS amps by definition require huge amounts of negative feedback >which we know compromises the source signal by introducing phase >anomalie's, is that still neutral?
John, IMHO negative feedback in amps that are well engineered is inaudible. I fear it has been used as a marketing tool more than anything else. I say this after having heard amps with feedback and without. My favorite amps that I own are:
(i) A SET 45 with no feedback
(ii) An ASUSA SET el84 ap with feedback
(iii) The zen amp with no feedback
(iv) The ASL K-502 kit 11bm8 PP tube amp with negative feedback.

>Could these effects derive from impedance mis-matches rather than >circuit design
Yes John, they could. My speakers are SET friendly, so they really should be SS friendly. However, and i may be wrong on this, most speakers are not SET friendly but only SS friendly. And then therer are those that are just plain unfriendly.. ;-)

Hope this helps.
-akhilesh