Now I realize why my PMC speakers, although rated to
go down to 35Hz with a puny 4" woofer can sometimes sound a little
forced when pumping out huge chunks of bass.
That is always the challenge with bass from small boxes it doesn't matter
whether it is PMC or another small ported design with impressive bass - they
all give up some clarity or quality in order to get big sound from a small box.
If you accept this and you prefer the quality/clarity then you should look for
either
1) very big box speakers (where the bass is accurate as well as extended)
2) for smaller box speakers without the bass extension (sealed boxes tend to
be more reliably well behaved in small speakers).
(there are exceptions to these rules but they are few and far between)
BTW - Harbeth are midrange magic! It would make a great choice. I would
add, however, a big PMC with a 3 inch dome mid would also be magic and
give you plenty of bass to boot!