Nice interesting post. Im doing something very similar and Ive been through about 25 amps or so comparisons, except for using the on axis bookshelves full range, I also try each amp on a sub.
I will say that I also grabbed the Zapco Studio 2x SQ from world wide for $65 as well. I was also somewhat underwhelmed by this amp as well. The amp I was comparing to a/b was a zapco studio 100 from 93 so it was very similar power. It was fine, but no where as clear and detailed as the older zapco, or the new full range class d amps from zapco. It was very easy to swap my into my car playing tweeters from 2.5 up and it was not as nice as the old zapco studio 100 so I am not using it and put the old studio back in.
The 4 channel st-402 sq class d sounds as good as my favorite amps and superior to the a/b sq amplifier to me and the others Ive tried. However, its not a bad amp, just not outstanding.
I will eventually get back to posting and write down all the notes on the various amps Ive tried. I will briefly share my favorite amps I have a/b tested on full range high end booksheves and the JL cp108 micro sub which is my testing sub I also listen to in tests. So typically 2.1 but if the amp is a 2 channel, I run it full range for a few work days, then move it to the sub and put another 2 channel on the full range bookshelves.
The pioneer d8604 was a fine amplifier as well and great deals. Most amps dont sound significantly different and if theres nothing remarkable, I didnt mention it. I have lots of amps around and tried many over the past 3 years I have been testing these while working.
I think now the Full range class d's from Both Kenwood and Pioneer are great budget choices and sound very similar to amps costing lots more. The pioneer, kenwood, and polk audio PA20002 all got the same score on full range. Same score as the Arc XDI.
I prefer Kenwood to pioneer regarding full range class D (treble response) but I would say they are very similar in "SQ" to the Zapco Studio SQ a/b series. Just 60 watts can run ut of steam fast if not on mid/tweeters. Zapcos F/R class d's sound much more enjoyable to me than the less expensive a/b SQs.
My personal preference is class a/b on the tweeters as I get an "airy" type feeling with good ones.
Overall, Id say all amps do sound different but it is very small differences between them. I agree with major differences are the pots used, rcas used and cosmetics when dealing with respectable companies.
Id love to see what you think about comparing the current Kenwoods class d f/r to the Pioneer.
The fact that you can get 1000+ watts 5 channels from both companies under 200 now is unreal.
Iím sorry I did read your post and then completely forgot to reply. Thank you for sharing your commentary and itís interesting to read about your findings on those Zapco and Kenwood amplifiers. I remember I had a real cheap 2 channel Kenwood back in the day and swapped it for that Alpine 3548 I used in my tests and was struck by the improvement in sound running a two way front stage. I then tried the same test but this time bridged driving a JL Audio microsub and the Alpine was noticeably more powerful sounding. I guess Kenwood has come a long way since those days.
I kind of want to do this again but this time get some DIYMA members involved and post their commentary. That might be interesting and of course it would have to be a blind test.