When I had a Mark III, I enjoyed the RCA EL34/6CA7 large bottle tubes. That ended in early 90's when Mesa no longer offered the RCA tube.
One set of tubes I did like in my Mark V were the JJ 6CA7. Those were sweat but did not last very long (2 weeks at best).
Gold Lion KT77 are awesome tubes and worked well in RA100, Mark V (sinister) but did not favor them in the Roadster.
SED EL34 were best tubes I have for the RA100. Just did not impress in the Mark V (could not try them in the MKIII since I sold it long ago).
The Mark IV that I had was similar but not quite a Mark III. What really sounded robust and by far the most dark grind was a pair of TAD6L6GC-STR and a pair of SED 6L6GC. Talk about complex harmonics

Perfect for Heavy Metal.
Most of the available EL34 in current production just do not seem to have that iconic sound. Perhaps the Mullards are close. Tung Sol is not bad. SED EL34 just had that sweet tone, saturated extremely well (best tube I ever had in the RA100). The only issue I had with the SED tubes (EL34 and 6L6) that were manufactured more recently (prior to them stopping consumer products) is tube rattle. EL34 were the worst. Other than that, very rugged and durable.