Pass Labs x250.8 vs tube amps

I have a beautiful system. Lumin X1 > BAT Rex 2 pre > Pass Labs x250.8> Sound Labs M545 (ESL) in a tiny room (9x 11 ft).  This system has the most glorious SQ: smooth, detailed and powerful , along with great bass. I would say this sound equals or betters any ultra high end system. HOWEVER, I have a problem. My system takes 6 hours of music playing to sound this good. I had X250.5 and I had the same issue. Leaving it on idle overnight doesn’t solve the problem. It is 15 W class A, so it generates quite a bit of heat, and yet I have to still play music. Because my room is small, I don’t crank up too much- maybe this is the problem, but the temperature doesn’t complete stabilize for 5-6 hours. The top reaches 107 deg in 2 hours, but the front plate doesn’t reach 107 until the 6 th hr when it starts to sound glorious. I have never had a tube amp and I am wondering if I can reach that thermal equilibrium and the glorious SQ faster with a tube amp? I worried about the heat generation with a tube amp in this small room, but X250.8 generates 450 W just idling. This is no worse than tube amps like Audio Research 150 Se, which I demo’d and was impressed by. BTW, I need power in the amp because of the esl speakers.
What do you all think?

Some tube amps , heat might be a issue, with my Defy7, 12 power tubes, heat has not been a problem   in summer months, 
And remember fall temps can make rooms chilly at night, , winters can be frigid, This is when  a  tube amp can be of an advantage.  

I think part of the issue as you have noticed might be because it is a class A amp and they are designed to handle the excess heat and once they reach a certain temperature they really start to open up.  I have a pair of Cary 211 AE amps soon to be FE. They play in class A up to about 75 watts which is more than sufficient to drive my Tekton Design Ulfberhts which  are 98 per cent efficient. The amps sound very good from the start but after about an hour and a half of listening when they get hot , they reach a playback level that is simply wonderful and  similar to what you are experiencing with your Pass Labs amps. 
 I also have a pair of Odyssey Audio Kismet solid state amps and they barely get warm even after several hours of playing but they sound the same from the time they are turned on to when they are turned off. 
 If I have company coming over for a listening session , I will start the Cary amps about an hour earlier to get them at least warm. The downside to this is tube usage but the upside once they get hot is more than worth it.
So in this long winded response is that depending upon the tube amp, you may experience the same issue with them. 
I had a similar issue with my system and it turned out that the pre-amp needed the long warmup. One time try turning on the pre-amp 6 hours before your listening session and see if that makes a difference.
I have a similar issue with my pass int 60. Sounds ok at startup, but takes maybe 3 hrs or more of playing time to sound its best. I just put up with it.