AFAIK, there's nothing to "measure" quality besides opinion. You can measure loudness, but that's pretty useless as a quality indicator. I could say Mercedes is a better car than Hyundai, but if mpg is your primary concern, maybe the Elantra is a better car for you. It depends on your application.
I actually *own* a Plus/2 (12.3) and Ultra (TV12). The Plus/2 is very loud, but sloppier than the ultra. The Axiom and the (TV12) Ultra are a virtual tie to me in SQ. YMMV. For HT, any of them would be fine and if output is your primary concern, the plus/2 is a fine choice.
Have you ever heard a Plus/2? If i had to do it all over again (with the Plus/2) it'd be a Hsu VTF3-HO w/Turbo.