Transatlantic Power tube problem?????

I have a Ta-15 with this problem, the volume drops after some time playing. It happens on the two channels. If I put it in standby mode and turn it on again the problem goes away, but if I continue playing, the problem happens again.

I've changed all the preamp and power tubes, but the problem continues. Does anyone know what can be happening?

Thanks.

Well, this is an update to my previous post.

I've noticed that the problem only happens in Class AB mode, 25W. I think it can be a BIAS problem. I have two sets of Mesa Boggie el84 tubes.

The one I bought new to put in the TA-15 causes this problem in 25w mode, but if I put them in a Mini Rectifier which I also own, It works perfectly. These are grey coded.

If I take the tube set that came with the Mini Rectifier (grey code too), and put them into the TA 15, it works well.

So it seems that it is very sensible to the tube set you put in it.