Possible output tube problem?

I just bought a Triaxis and a Stereo 2:50 power amp. I started having a problem where the amp would suddenly lose gain and volume. At first, I thought this was caused by a faulty George L's cable. Later, while I was playing, I noticed the volume fading in and out. I'm guessing it's a tube problem. The units are new, but the amp sat in the store for awhile... The Triaxis came straight from Mesa. My question is: Would this type of problem be more likely caused by a preamp or output tube? I've eliminated all other possible causes (i.e. guitar, cables, etc.) Any advice would be appreciated!

It is hard to say with tubes. It is just as easy to have a preamp tube problem as it is to have a power tube problem. Tube problems are just something you have to live with owning tube gear. What I would do is go ahead and get a spare set of tubes because you will want a spare set lying around in case you need to replace something in a hurry. Then I would swap out the sets of tubes choosing either the preamp tubes or the power tubes to start with but taking notice any changes. That having been said swap some tubes and play your gear for a little bit. Then, if your problem is gone then you will be able to determine which set it was. From there if you want you can isolate the bad tube. This will at least get more response from Mesa if your amps are still under warranty.