Bass leaves after amp warms up?

Fish, if it were me, with those speakers i would not use any integrated if I had the option in any case. I think you would find the right dedicated power amp to be a clear improvement all the time. That's why I suggest the option of buying/trying one. My assumption is that you would keep it if it resolves your problem and sounds better. If not, then have the option to return it. That's a good and fair practice anytime one buys a product in good faith. If it does not work out, then you always should have the option to return within a reasonable time period. Nothing wrong or unethical about customer satisfaction.

Now what to do with teh current amp in that case? that would depend on if it is working as designed, which it may be, or defective in some way. Testing it with an easier pair of speakers to drive would be needed to determine. If it can't drive an easy speaker load, then it may be defective, as opposed to just not up to the task of driving the PSBs specifically to their full potential.

I agree with Mapman regarding a power amp. One option is an Emotiva; order online with a 30 day audition return policy. But first let's get the SPL test results.

In order to use a power amp that means purchase of a preamp as well,correct?
I was interested in Bryston new line of pre/power & was informed by Mike there is no difference from the B135 integrated (which I owned)

Sorry, Mapman. I misunderstood. I thought you were suggesting to get one just to test the theory. My bad.

In order to use a power amp that means purchase of a preamp as well,correct?

No, not necessarily. As I suggested previously, you could borrow or purchase a separate power amp, and drive it from the pre-out jacks that are provided on your M6i. You would then be using only the preamp section of the M6i, together with the separate power amp.

Regards,
-- Al