Well then, I agree with Lon. Especially since you are SOL without the rectifiers, why not order the Valve Art 274B from cryoset to get back into the music?
Then if you want to try for more volume.....You likely have 6N1Ps as input tubes (probably has a diamond on the side with a number in it). If so, by changing to a 7DJ8, and if your MKIII is like mine, you will get more volume before clipping (distortion). You could use 6922, 6DJ8 or 7308s too for the same reason, but for the money 7DJ8s can be really good, costing less than a number of new 6922s, and typically way lower than good NOS 6922/7308s, though I really like 7308s too. I think this is a nice, warm but open 7DJ8 that wouldn't hurt too bad to try the tube type.
http://www.tubemonger.com/MP_MINT_NOS_Tungsram_PCC88_Jan_Feb_MAR1971_Hungary_p/7...In my experience, these guys stand behind their tubes if they have problems also.
As Lon suggests, I think I recall hearing from Ron @ cryoset that we might expect 3-5000 hours from power tubes....that starting to sound "sluggish" is a good indicator of losing it. I am using Genalex KT66 also, but in my MKIV. If you check them out, it may be worth asking Ron at cryoset whether he thinks the Tungsol KT66 or the Genalex will give you more volume. They are both nice, though I prefer the Genalex too, but they draw quite a bit more current, and I don't know what this means in terms of volume. I was not paying attention to that when I changed to the Genalex.
Maybe someone else here can speak to that.