You really shouldn't get a fizzy sound from the XXX with any tubes unless the amp is not set up properly. First of all, make sure the master is cranked and use the channel volume to control overall volume. Dampening switch should be set to loose and the effects loop controls set at 12 o'clock to start. This is not set in stone by any means, if nothing else the sound level with the effects loop on and off should be about the same. For more information on setting the effects loop controls do a search on unity gain or just turn them until you get a sound you like. I'd tell you what mine are set at, but I can't remember.
Finally, set all the EQ controls at 12 o'clock and adjust as needed.
I've had my XXX for over two years and have read all the threads here and on harmony central and have spent a lot more on preamp tubes than I care to admit. Most of which are sitting in a box unused. I think that the preamp tube obsession is overstated, as it is with many amps.
Here's how I hear it: Some tubes are high gain, some tubes are low gain, and others are neither here nor there but work well in V4 and V5 to keep things quiet.
In a high gain amp like the XXX I like to use EH 12Ax7s in V1-3 and a JJ 803s in V4. Other JJs work well in V4 also. I recently changed out the 7 year old Sovtek LPS in V4 with the JJ 803s and was surprised that the amp became brighter and louder....in other words I think the Sovtek must have been close to dead as I've never heard such a difference from a change in V4.
I run my 6505s the same way, starting with EH and ending with JJs. The 6505 tube layout is a bit different than the XXX, as it has an extra preamp tube and I think the order is different. Both of my 6505s came from the factory with three EH and two JJs. Probably not a coincidence.
A lot of the harmony central guys disagree with me, but a few do agree: Low gain preamp tubes are like a vasectomy for the XXX. If you want a low gain amp, don't buy a XXX in the first place. Buzz and fizz aren't from the preamp tubes, they are from the way the amp is set up.
I'm sure I will be told where I am going wrong!