I got one of these not through massdrop, but as a gift. I didn't know ANYTHING about tubes whatsoever prior to that, so I had a lot of the same questions. I'll do my best to answer.
"What are each of the tubes?"
There are two tubes. The larger one in the back is the power tube, whereas the one in the front is the actual amplifier.
" Are there size designations I need to know? Or model numbers include the size? "
There are model numbers actually printed on the unit itself, immediately beneath each tube socket. The power tube is "6AS7", and the amplifier tube is the "6SN7". As far as I can tell, as long as you get a tube that is a 6AS7, it's gonna fit in the 6SN7 socket, period.
" How does one go about shopping for different tubes to tube roll? "
Research and ebay. The default tubes aren't bad by any means (at least for me), so at least start with them. (Note: my tubes were a bit corroded upon receipt so even though they worked and sounded great, they would have failed sooner rather than later.) Also, the keyword you're looking for is "new old stock", or "NOS". This means that the tubes are "new" in the sense that they've never been used, but have been "in stock" (on a shelf) for a while. Like 40 years a while.
To replace my tubes, I did a fair amount of research. I got myself an RCA Military 6080 for the power tube, and an Electro-Harmonix Gold for the amp. My research suggested the EH was a very good choice, whereas the RCA military tube....I probably could have gotten something better, but oh well. It sounds nice, but bass isn't QUITE as punchy as I'd like. But it still sounds fantastic.
" Do you get both sizes at the same time? "
You mean both tubes at the same time? Yes, they both come in the box. You mean "do I have to replace both at the same time"? No, you don't.
" Do you just replace the big one? "
Some final notes:
- This thing runs HOT. It shouldn't be an issue, but just don't grab the tubes when they're hot. I have a small USB fan (a quiet one) that is blowing on the side of this and things are MUCH cooler.
- Sometimes when you get tubes, particularly the preamp tube, you might hear some hum/buzz. This does NOT mean you have a bad unit, or even a bad tube! When tubes haven't been used for a while, air leaks in VERY slowly. After a few days of use, or a solid day of burnin, the hum should dissipate/disappear. (Basically you're burning off the air. It's complicated and I'm not good at chemistry.)
- This thing is awesome, and make sure you have a good audio source! Your 30c DAC in your phone is NOT a good audio source. If you're buying a tube amp I assume you already know this, but it's worth mentioning.