How are tubes to be purchased in general?

You need a auto ranging multimeter to check the bias on the power amp tubes. If your lucky whan a tube starts to go you will hear a little distortion . With the multimeter you can see which tube has a bad bias reading, which cannot be corrected with the bias adjustment. Then you replace that tube and your set.

If the tube blows it may take out a fuse or a safety resister depending on the amps design.If the resister goes you need a trip to the shop to replace it.

I prefer tube sound so I thinks its worth the extra cash, and hands on attention... for the next step closer in Musicality.

I agree with Weiserb. I recently blew a tube(while biasing) and thought a new matched quad was in order. But as stated above one tube, some biasing, and your back in business. I did manage to find another quad of the same tubes(JJ's) so now I have extras for backup. The attention Weiserb mentions to me is very important. Best of luck. (BTW I did have to replace a resistor).

I'm not familiar with the biasing process for Rogue amps but some amps have a bias trimpot PER TUBE (CJ) which makes tube matching less critical. My ARC VT100 MKII amp had just one bias trimpot PER FOUR tubes. In this case tube matching is more important.

You can buy matched pairs or matched quads from most tube dealers or just a single tube. If just one tube blows, I would probably buy a matched pair anyway and just keep the spare handy. Over time, you'll probably own several sets of tubes anyway! I have KT-88's and 6550C's that I like experimenting with. It's part of the fun.

As for expense, in all honesty I have never had a tube blow or wear out on me yet but I've only been playing with my tube gear for about 1 year. Even if you had to replace ALL your tubes in 1 year (not likely) thats a small investment compared to the quality sound you'll enjoy with tubes.

I've tried ARC VT100 MKII, Conrad-Johnson Premier 11a, Classe CA-200, Classe CA-300 (bought and sold used). So far I like the CJ the best. And so far I still like the original 6550C tubes. Have fun!

Agree with the posts above. Nice to measure the voltage (bias) running in your tubes - get a meter. The only time you are going to run into trouble is if your tubes are running hot and some amps change over time. Not like Sand amps where you plug them in and never think again for 10 years. Get to a local club or post on the audioasylum.com tube section. Someone will have your amp and will be able to tell you just how to take readings and what they should be. I think you will find the audioasylum site invaluable if you are new to tubes.
Good Listening!