You can pick from many brands of tubes as long you you stay within the specific types.
(Note your Must have the amp BIASed if you replace the Power tubes or you may damage the tubes and the amp itself if done improperly.) Also be sure to Mark what tubes came out of what socket as it can matter if you get them mixed up. you can use a marker without damaging tubes.
You have two areas of tubes in this amp.
1.Power Tubes -These are the four Larger tubes you see in the back of the amp.
To note that it is ALWAYS best to buy a Matched SET of Quad Tubes for the power section. The only time you should replace one would be if it is the same brand and of the same type of tube.
Most player prefer brands of JJ's or Tung-Sol Wing-C Electro Harmonix etc.since the factory tubes tend to be Glassy and fizzy in tone being of Chinese production.
2.Pre-Amp Tubes - These are the smaller tubes that are either defined as 12AX7 or ECC83S and you should have Four Pre-Amp tubes in this amp noting that the one next to the Power tubes is known as an PI (Phase-Inverter) tube. To note when removing tubes bleed the amp down powre off and depress the Bear Claws (the metal things that hold in the power tubes) and DO NOT PULL FROM GLASS grab from the base of the power tubes so you don't break the tubes/seal. Rotate the tubes out rather then pulling them straight down. Use a slight rotation and they will come out even rather then fast bending pins.
On the 12AX7's you remove the metal retainer caps and the Pre-Amp tube you do grab by the glass. Should you have trouble gripping use a piece of that non-slip cabinet liner like a coozie and it can grip the glass extremely well.
Your choice of tubes can dictate the overall tone of the amp. This amp is a Modern High Gain from design so if you have to much gain look for tubes that are less gain in the preamp section. Keep in mind the phase inverter tube when buying.
Reminder that ALL TUBE AMPS CONTAIN EXTREME HIGH VOLTAGES THAT CAN HARM OR KILL so if you are not tech savvy or not comfortable in doing this please take it to a qualified tech.
The Pre-Amp tubes are what gives the amp specific tones low mids highs gainy or clean. The power tubes in the EL-34 give the amp that British breakup sound. Gravely. Gritty
The 6L6GC allow you more headroom as they say.
On Bugeras Tubes all tubes are marked with a letter designating the tube.
12AX7A-B-C the A can be found between the pins these are the factories why of determining what goes in what slot.
On a last note most players have to experiment on tubes to find the specific tone they are looking for. Should you never find that tone then the next thing is to find a amp that sounds as close to the tone you want from the store and start with that again doing tubes etc.
Please research Power tubes and Pre-Amp Tubes on the internet it take reading...
Enjoy..