It would be easy to just list tubes that I like for various reasons, but that would not be benificial to you. A power tube’s sound depends on a lot of other factors. Even though people are following the same basic schematic, your choice of speaker, power supply, output transformer, and preamp tube will affect the power tube’s sound. It would be troublesome to get to technical in this area, just test out the tubes you can get a hold of easily. Some power tube might sound bad in your amp, but sound great in someone elses due to supply voltage and sag, or due to their choice of preamp tube or speaker. Since adjusting you power supply, or sampling various speakers is quite a hassle, and expensive, then try a couple of different preamp tubes with each power tube. Some might sound horrible with one brand of 12ax7, but sound amazing with another brand due to the fact that companies nowadays will lable almost anything an 12ax7 regardless of their actual tube curves. Same goes for any new power tubes as well, just cause it says 6l6, the curves may be well off the mark. You can also try 12at7, 5751, 12au7, or even a 12dw7 among many other preamp tubes. Also don’t get hung up on price. Just because some tubes cost a fortune, does not mean they sound any better. You can even change the cathode resistor value to adjust your bias point if you want. This will also affect the power tubes tone. It is all about trial and error and most importantly, trying to have fun.