For me I get the best results using the current reissue Tung Sol 6SN7 tube, matched tubes should be readily available, and at a reasonable price. I also strongly suggest you get matched pairs of the 6C33-B tube for spares, it will come to point where the tubes will drift apart causing the fuse below to blow. The tubes may still be fine, all you need to do is find another tube that matches either of them. Here the closer the pairs are matched the better the amp will sound.
A more important aspect of this amp to consider may be the age of your amp, and to get the most out of this amp would require you to change all the electrolytic caps and the four .22uf oil caps. If you are good with a soldering gun and own a good desoldering gun, the job is not too difficult. There are currently new electrolytic caps that are just coming available that are simply amazing. If the four Jensen .22uf oil caps don't have the epoxy ends they usually start to fail or their performance starts to drop around the 5-7 year mark. The Jensen caps used are the entry level aluminum foil aluminum tube ones, to greatly improve performance, durability and sound signature, the copper foil copper tube 1000V cap from Jensen is my favorite capacitor.
Their are many aspects of this amp that one needs to know to simply operate it and to greatly improve it's performance and reliability, definitely not a plug and play unit, but once you know it's idiosyncrasies you come to realize how great this product truly is, a product truly ahead of its time.
