In my opinion, the best way to get more sound out of any amp is by changing or adding to the speakers.
Think about it. 6550 tubes, as used by Sunn, put out about 30 watts each. You might change to KT88s, but what will that gain you? Let's say you get up to 80 watts. that's an increase of only 1.25 dB. Odds are you wouldn't even hear it. Add a second, identical cabinet, and you gain at least 3 dB. Change out the drivers from drivers with, say 96 dB sensitivity to ones with 102 dB sensitivity, and you gain 6 dB. Those changes you can hear. Another thing is to mike the cabinet, and put it through the PA. Or - and I admit, this is a dangerous concept - the rest of the band might turn down. Most bands play so loud that the audience can't converse. If you're playing at a dance or a club, they'll want to. If you're playing concerts, you'll want to mike the cab into the PA anyway. Use your amp to get the tone you want. There are other ways to get volume.