Bass leaves after amp warms up?

They were all integrates

One more time...Go to Radio Shack, then test the SPL.

I'm betting the SPL test when it happens will be inconclusive.

Fishing, do you have a CD with frequency-specific tones you can use, like one of the old Stereophile Test CDs? One-third octave warble tones would probably be good and you want individually measure enough of them to cover the range from deep bass up through the midrange.

04-19-13: Fishing716
I have a portable cd recorder & plan to record when the bass is there & not-maybe hear the difference from the cd I make.

The following was written before seeing Drubin's post just above, with which I agree:

What would really be ideal, if you can arrange it, would be to mount a good quality microphone on a stand at the listening position, and record its output when the problem is and is not present (using a suitable microphone amplifier together with the CD recorder, if the recorder doesn't provide a mic input).

You could then upload the two files to a web hosting or file sharing service, and we could analyze them with audio editing software some of us have (including me).

One reason that may be more meaningful than an SPL measurement is that unless you use a CD or other recording that provides test tones, in conjunction with the SPL meter, the SPL measurements may not be able to distinguish between a reduction of bass frequencies, and a reduction of volume that may be perceived as a reduction of bass as a result of the Fletcher-Munson Effect.

Regards,
-- Al

do you have a CD with frequency-specific tones you can use,like one of the old Stereophile Test CDs? One-third

Yes I have Stereophile test cd #3