Tube stereo sounds -smaller- after being on

You will have to evaluate each of the many components of the system in order to figure this one out, but the one thing that could affect all is your AC supply.  Is your line voltage constant?  Otherwise, this is a DIY question.  Got a meter?  Check AC voltage out of that step down transformer as it heats up.

I myself have been Wed to Tube Amplification for nearly 30 Years.

I don't delve into the Circuitry as all Items in use are Bespoke Built, so I leave this to the EE's whom I have trusted. 

I have done and do Tube Roll, I am supported with my Valve Purchasing and have a Testing Service at Hand, all my chosen Valves as a result of Tube Rolling are Matched and Optimal in the Test Measurements, some pairs are even above Manufacturers Spec'.

Certain Tubes are now discovered for their ideal placement in the circuitry and there are a few tubes, I would very much like to play with as a Journey of discovery but have not, as Tube Rolling will be quite some expense.

During Tube Rolling, I have had in the Line Up, Vintage, NOS and Modern Versions.

My finding with Certain Tubes is that there has been a initial impact that has been quite attractive and certainly a comparative attraction to another Tube that made a good impression.

I have used Tubes that have initially impressed, and have improved as the Tube has a extension of use. This good impression has been followed by a need to remove the Tube with immediacy, there has been a change to the sonic after  a certain period of being heated that has caused a detrimental effect to the SQ. In a few cases to the point, it was a very concerning sound being produced, filled with unwanted distortion.

I can't help narrow down your own Trouble Shooting requirements, as there is not too much info' on offer to explain how the approach to fault finding has been undertaken.

My experiences has led me to Swapping Tubes across Channels, it is one of a few  commonly seen methods.

In your case, with Two Systems, swapping Tubes across systems might also be an option to see if the unwanted condition manifests with Tubes that are satisfying in another device. 

On the plus side you are fortunate not to be dependent on one system only, music can be replayed at your leisure and time is available to tinker around with the Second System. 

A couple of suggestions.  I'd first swap out the tubes and see if that made a difference.  Tubes heat up and generally the changes are good but not always.  Not saying this is it but an obvious thing to try.  

On your power converter and electrical system feel it and see if anything is getting warmer than you would expect.  As copper heats up its resistance goes up putting it into a positive feedback loop.  So it could be something in your power circuit is undersized.   

I have a 240V tube amp made in Europe.  I run it on 240 from my home panel.  You could consider installing a 240V outlet for it and eliminating the power converter.  I did experiment with my amp and ran it successfully from a transformer style power converter.  It was hard to find one that didn't hum.  I also oversized by a factor of 5.  If your amp is 200W, buy at least a 1000W rated transformer. 

Jerry

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.