absolutely silent. Not a sound. I had one dedicated 20 amp line for my mono blocks. Transport and DAC. Xa 100.8 and even my Xa 30.5 were silent.
I now have 3 dedicated 20 amp lines ;one for each mono block and the third for both the transport and DAC. i am very sensitive to hum and hiss or any extraneous noise but i hear nothing from the 100.8. i even kneeled , held my ears close to the amplifier and hear no sound . just completely silent. Hope this helps.

My XA100.5s are silent like a tomb.

I also tried the XA100.8s in my room and they were very quite.

I may have the chance to purchase a pair 100.8 but I am quite worried about the warm up time... is the 1-4 hour from cold start, or from standby?
Do the amps run warm or hot in standby, any idea what there consumption is in standby mode? Thx.

Well, a quick check at the Pass Labs website shows the amp draws 3.5 amps at idle or 420 watts.

https://www.passlabs.com/amplifier/xa100.8

You can't be concerned about your electric bill if you want pure class A bliss.

@FreddieFerric well, it’s not so much the consumption alone that would worry me, more the practicality. My system is setup in my living room, no airco... so if the XA 100.8 are warm to hot even in standby mode, they could be problematic in summer heating up my room too much... I could switch them off completely of course, but I read somewhere they need a day to sound their best, when powered up from totally off... No idea if this is really true?How long do they take to sound really good from standby mode?

It is not hot in stand-by...

Stand-by
 

0.5W each in stand-by

Thanks you guys for this info! How long before they sound good from standbye?

I hear you. Hot amps in summer months can be problematic. I used to run tubes in my small listening room, but down here in the deep south, even with air con, it was just too much.

I went from a ARC GS150 to XA100.8 and they throw off a lot less heat.