Here's what I've discovered about the noise thing so far:
I'm sure there are some better tubes that will cut the hiss some but I don't think it's primarily a tube thing. Seems to be a gain staging thing with the way the different channels are set up. You can't just crank up and go with this amp. And it's different in the 5 watt setting and the 25 watt. Takes a lot of exploring. The gain/eq/master relationships are very important and extremely sensitive to small changes.
The most offensive problem on mine was on the #1 Clean. If you crank up the eq knobs here, especially the treble, keep the gain below about 11, and then crank up the Master to get it loud.....HISSSSSSS. But... if you keep the Master way down, keep Mid and Bass way down, set the volume with the Gain and Treble, and then barely ease up the Master, you can get a good loud clean with little hiss. Clean will take a lot of Gain without distorting if you keep the eq settings low.
#1 Crunch.....if I do this one right I can get to 12 or so on the Master with almost total silence. I love this channel. I spend most of my time on Channel 1.
#2 Blues/Burn. You expect some hiss/noise for cranked gain type stuff and #2 isn't that bad unless you start cranking the Master. My buddy's Marshal DSL 50 is noisier at around the same gain/volume levels. He comments on how quiet mine is.
I never use the Master over about 9 10 oclock. Anything above that, except for on the #1 Crunch, just turns into hiss/mud. One of the keys for me and mine: Set volume levels with gain and eq, with those as low as possible for the sound you want, and then fine tune sparingly with the Master.
Seems to me like the gain on this amp is just kinda set to HIGH at all times. Wants to just scream with little juice, especially when just using the internal 10". Thus, the hiss when cranked. It doesn't NEED to be cranked....it's cranking at idle almost. Using the 4ohm jacks and going to outside cabs seems to give it a lower noise floor and allow for more cranking/headroom.
I'm coming up on a retube shortly. Will be interesting to see if new tubes affect the hiss level as some have said. The Webers I put in mine really get this amp singing too. I think speakers make more difference than tubes.
Just a few observations. It's always interesting to me to read how others have approached the hiss problem with the Express. I think some gave up too easily and just got rid of it. I'm glad I stuck with mine. Great little amp. Records very nicely too.