You are welcome. Re the tubes. The 5751 is a lower gain version of the common 12ax7 tube. Some of the best 5751 and 6FQ7 tubes were in fact made in the USA so assuming those tubes test well and are functional, I’d keep them for now.
The four output tubes that you currently have are 6550 tubes made by Svetlana. They are decent Russian tubes. However, current premium made in China tubes have really improved in quality. Both myself and my friend who has the 11A use some of the top of the line tubes made by the company Psvane. We both use the KT88, more of less a direct equivalent of the 6550.
There are generally more KT88 than 6550 tubes made these days.
Russian tubes generally currently come out of the same factory I believe, and are branded various names like "Gold Lion" or "Mullard" after the names of tube makers of fame from the past but they are nothing like those tubes.Having used some of these USSR tubes in the past, I generally have preferred the current premium Chinese tubes. All personal preference of course.
There are also what are called NOS (New Old Stock) , old production tubes made in the heyday but now at very lofty prices due to their rarity. So there is your basic tube primer related to your amp. Have fun. Hope I have not overwhelmed you!