I've dealt with the hiss on the clean channel of my 5:25 10"er thusly:
Turn the Main way down, 9:00 or below, and use the Gain primarily as the volume control and then the Treble, turning Mid & Bass way down. The gain structure is different on the clean channel.....seems the Master sends too much juice to the internal speaker with the Master, Treble and Gain cranked up. This is the only thing that gets the hiss to an acceptable level when using only the internal speaker.
Bypass the internal and use external cabs with bigger/more speakers. Can get more Master level that way with less hiss. With a 1X12 and 8X10, the clean is nothing short of beautiful. Quiet and HUGE...almost 3d.
Use the internal with an external cab and attenuate the output to the internal 10" to let it drive the external way harder than the internal speaker.
Don't try to make the 5:25 a clean-volume-monster like a Fender Twin. It just won't do it. Find the sweet spot at a reasonable volume level and ENJOY! If you need huge clean volume, get a Twin.
For me, using these methods keeps the hiss level pretty much the same or less than any other tube amp.
I'll know in the next couple of months whether the Doug's Tubes retubing really works. Will post my findings when this occurs. I've still got the original Mesa(?) preamp tubes in there. Have gone through about 4 sets of power tubes so far in a couple of years. It's easy to tell when the power tubes start getting weak and it greatly revitalizes the amp when they're replaced.....but it doesn't really affect the hiss. The HISS is either related to the preamp tubes or just the inherent gain structure of the amp. Looking forward to the retube! If it cuts the hiss 60% like some say, I'll be one happy camper.