LP playback is lacking - any ideas?

Get rid of the turntable and buy a Linn Sondek LP 12. End of problem.

Are you using power conditioning? Dirty power can muddy sound.

Standing waves(bass) can make a system sound lifeless,small, congested. The gear you have is capable of at least 80%, of state of the art performance, if properly set up. Tube traps would be the fix, for that problem. DIY traps are very easy to make.

Could be anything -- dontcha love it? If the arm is set up properly, then I'd be tempted to look first at what is supporting your TT (you don't mention that) room as a likely culprit. Of course, if your CD player produces a similar kind of congestion, I might suspect your room.

Oops, strike the word "room" in the second line, above. I just mean what your turntable is sitting on.