Tubes can hiss. It's a common problem. You can try different tubes one set at a time (preamp tubes, power tubes). That's where I would start.
Hissing at idle through one amp but not another
So I inherited a wonderful Ayon Crossfire from my uncle and it sounds awesome but I'm having a hissing that must be related to the amp/preamp match but I don't understand why.
With my Lamm preamp and the Ayon with the mute on, the speakers are silent, when I unmute I hear a hiss from the tweeters, just loud enough to hear at the listening position 10 feet away with no music playing and also during quiet passages on very quiet vinyl. It happens equally with any input selected on the Lamm. It's the same level of hiss no matter what the volume is set to. I've swapped the cable from preamp to amp. No change.
And then, when I swap out the Ayon for a vintage Fisher SA-100, there is no hiss anywhere at any time. Completely silent.
This is killing me. What the hell is going on!!!