The Bias switch at the back gives the option of EL34s, I have just installed a set of Svetlanna 1999 valve from Watford valves in Englkand , these are old stock brown top valves and are supposed to be the best,
I have had a mk5 for nearly 3 years and found the tone good but the bottom end is very soft and flabby in my opinion, and you cant fix it with the EQ either. I play in 2 bands , classic rock and a pop band , but i also play a lot of blues stuff.
The first impressions of the EL34 valves is that the tone is much tighter, and channel 2 sounds better balanced with that low mid softness reduced and a harder edge. Watford valves also advised that there is a mid way valve i think mits a 6V6 which is somewhere in between.
All i can say is i think the EL34 valve is much tighter sounding which you cant hear from all the you tube clips that i have seen, the change is actually very easy to hear in reallife.