This is the first time I've ever posted on a hi-fi site but could really use some expert advice.

I just recently purchased a slightly-used Luxman SQ-38u froma Luxman dealer.  He promised me that the amp is essentially like-new and even offered the full 2-year warranty through his shop.  Given that it doesn't need break-in time (since it's not new), I was surprised by the sound.  The mid-range itself sounds really great.  The low-bass is also pretty good for a tube amp.  But there seems to be basically no low-mids whatsoever.  So, for example, with low notes on a cello, or bass guitar, you can hear the boomy low bass but not any defintion.  It's just a big hole in the spectrum.

Might this be a problem with the amp?  If it had a bad tube, would it cause frequency dropout like this or more obvious artefacts?

If I can somehow find some reassurance that the amp is performing like-new, then I would conclude that it's just a weird mismatch with my speakers (PMC PMC DB1i's).  But I don't want to go in circles demoing or replacing my speakers if the Luxman itself is not performing as it should.

Any thoughts would be much appreciated!  Thanks!