Loss of bass & full spectrum sound

My Girlfriend hears the same thing as I
My amp is brand new and already had 3 amps here with the same results

Today the bass was there then after about 2 hrs was gone.
I switched speaker cables from left speaker to right side of amp & vice vera and the bass worked again
It has remained as I write for about 2 hrs.
it can (and will)go at anytime
The result is always unlistenable
Right now it sounds vital
I have Called PSB,Audioquest & Bryston(I had B135 for months)now have Musical Fidelity M6i
These companies were reluctant to believe me or suggest a remedy.
My Dealer in Florida *I am in NYC also has no solution.
At least he allows me to trade recent purchases for newer products with little loss.
I am saving to audition PS Audio P-3 power station with return previlage in hopes that will solve the problem.

I would really like to hear from other members with similar issues

I worked in the audio industry for over 20 years, and even had upscale systems years before that time. Have helped many friends and other people solve just about any problem you could imagine, however this one is unique. You said you have used different amps and speaker cables, has this happened with the same pair of speakers?

Haunted. Your system is simply haunted. Get out while you still can.