Which tubes for Wavac MD805m

I don't own a Wavac but just know that all of these tubes have different gain factors and are not exactly interchangeable. For example, a lower gain tube can be placed in a circuit made for higher gain tubes, but you wouldn't be able to place the 12ax7 or the 5751 in a circuit designed for the 12au7 because it will not be able to handle it.

Gain factor
12ax7=100, 5751=70, 12at7=60,12au7=19

Thank you Cyclonicman. I have contacted the manufacturer and have been informed that the original input tube is the 12AX7. So now I can narrow my search on characteristics of different 12AX7 tubes. Is it worthwhile to go for the NOS Mullard or Telefunken tubes?

I don't think you can go wrong with either one. At one time, I had used both but that was in my preamp and either will deliver excellent quality sound. The Telefunken seemed to have sweeter high's, but both produce excellent mid range sonics. Nos tubes are very expensive and sometimes don't last too long, but I felt they were worth it. I am just not sure if the sound quality is as apparent when used in a power amp. I also found the Amperex bugle boy to be a good tube as well. Enjoy the journey!