i remember when i got my first valve amp about 5 years ago i had such a shock to my playing i'd been using a JEM7VWH w/Evo's into a Peavey Bandit 112.
I got a Marshall JCM900 and it totally changed my sound, everything had dynamics and it wasnt compressed and I could hear tonnes of mistakes in my playing..
Then I got a ENGL and that made me realise ever more.. then i got BKP's and once again.. realised, your gear really can hide your mistakes, i play a strat with VHII and Irish Tours and they dont hide anything.. and it's brought my playing on a mile... just wish it'd happened years ago.
but this is why a lot of people like EMG81's w/ 5150's etc.. coz they're a shed load easier to shred on than a set of Irish Tours into ENGL Screamer.. but i'll tell u if u can get it clear clean and tight on my setup it really does help.
rant over...
I'm not so sure how exactly a JCM 900 covers mistakes more than an ENGL and not less, but you basically said it right on.
The way I see it, if the clean tone is too compressed with the volume down, the pickups are useless.