Tube stereo sounds -smaller- after being on

First you have to figure out which piece of equipment is causing the problem. Fortunately you have two systems. You take each piece of equipment in the bad system, one at a time, over to the good system. When the problem tracks to the good system you have narrowed down to one piece. Then you can change tubes etc. and ID the exact problem. I would start with the amp.

The only time I've experienced what you describe is when I was using my vintage Pilot 232 (1959/1960 PP EL84 power amp) 15+ years ago.

After 3-4 hours of play the soundstage would gradually shrink/compress and the power transformer would become extremely hot by then.

The amp was all original (lytic caps reformed to spec) with only the ceramic resistor feeding the 5ar4 rectifier being replaced with a like unit.

When I powered it off a variac set to 110 volts the problem did not reoccur and the PT did not become overly hot.

Didn't check the A/C line voltage @ the time, but assume that it must have been fairly high.

DeKay

Thanks everyone for your thoughts. I only thought about the line voltage being an issue when I wrote the above post. The amp came with a Chinese made voltage regulator that doesn’t seem to be adjustable and shows 217 volt output on the led screen. I always thought that was pretty weird. But since it came with the amp from China I didn’t question it. As for other possible issues, the mixer, being Australian made, could also be in some voltage netherworld. It comes with a separate power supply set up for the US which gets very hot. I can take that out of the loop easy enough and see if that’s the issue. Plus I probably should just buy a different regulator for the amp. 

@yaluaka Sorry when you say Mixer what component are you referring to? A mixer as in pro sound? And you're using it as a preamp?

The mixer is a great sounding one hand built in Australia called a Condesa Lucia.