Tube stereo sounds -smaller- after being on

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. 

@yaluaka I don't see any reference to your OP or follow up that suggests you are seeking route to HiFi Nirvana.

I only see a request for ideas on a solution to resolve a issue with a sonic that develops that is unattractive and wanted to be overcome.

I see no reason for you to get rid of any of your equipment unless you feel the need is necessary. 

Your Systems, Your Home, Your 20 Years spent building a Tube Based Set Up.

I'm sure there is plenty close at hand to meet your needs perfectly, it does not have to be anybody else's ideas that are needed to be followed.  ,

@yaluaka I am fortunate like yourself to have a real love for the music.

I got to a place with Source (30 years ), Power Amplification (20 years) and Speakers (20+years) that I have been loyal to for the years bracketed.

These are the foundation to delivering to me a musical encounter just how I like it.

The Mechanical and Electronics of the Vinyl Source has evolved over this period but the Source Material is collected since the early 80's.

I love to see creativity around HiFI especially the Vinyl Replay and get caught up in a variety of not too commonly undertaken experiences. This could easily be interpreted as I am a 'equipmentphile', but no, this is just an extension of my enjoyment of musical replays and is also a social activity as it takes getting out and meeting with others and in some cases witness the end product of an individuals skill set.

I was sure and am now quite sure you are a individual, who is without snobbery towards how a musical encounter is to be created. I would have wager on, there being the odd component in a Device you are using, that is known for its sonic character, and the idea of parting from it will be a real bitter pill to swallow.

For me the component is Valves, but I am not a under the hood delver, so other parts are out of bounds unless an EE is at hand as a guide.