Intermittent bass

Can anyone pose a reasonable solution for a long existing problem?
I have excellent audio equipment
Bryston B135 integrated amp & matching cd player,PSB Synchrony II speakers + audioquest comet biwire speaker cable & Rega P5 TT/dynavector.
The problem I'm having is I get a really deep satisfying bass on all sources then at any given point the bass becomes dull & undefined.
I've checked & even replaced many of the above items but the problem persists
No one has been able to point me in the right direction..
Has anyone experienced a similar problem or guess why bass weight is changing?
The weight on the cartridge could be the culprit. Too lihht may rob you of bass. It will show more on loud passages and possibly certain points on the album due to tracking.
Just a guess,since I never had that problem.The first thing that comes to mind is a bad connection.Since you biwire,I would play some bass heavy music and wiggle the cable at the binding posts on your speakers.One of the posts may be bad,the cable itself,a connection inside the cabinet[hope not]Next maybe switch the cable ends so the ends for the bass are now passing signal to the mid/tweeter and vice versa.Try each speaker one at a time,then try the amp end.That's a start anyway.Best of luck to you.
Thanks for your assistance
I tried all those ideas though
Having checked the cartridge and connections, the only other thing I can think of is some kind of driver damage that escapes your view. It could be a small tear or rip that runs concentrically.

I hope it's an easy fix for you. Good luck.

All the best,