This is a very interesting thread. I can understand why a more powerful amplifier in the same series (7B cubed VS 28B cubed for example) could sound better at high SPL's , particularly with a speaker as capable as the PMC's. But I am perplexed that you would experience such a difference at low volumes while only pushing a watt or two. I would have suspected the two amps from the same series to sound very similar at low output levels ?
Cheers,
Scott