You can have it gone through by a good tube amp tech and brought to spec, even pump up the juice with a solid state rectifier.
But you still have a 120 - 150ish watt amp. I use a 2000S with a pair of 2000S cabs (when I feel athletic!) and it's as loud and thunderous as I ever would need, more than loud enough to infuriate any sound engineer.
I suppose it depends on how loud you call "loud" is; but to get it to what I would call serious loud with only one 4 X 6550/KT88 amp, be it Sunn, Matamp, Orange, Hiwatt, etc., you will need to add a component...
A big sound system.
I don't know much about doom except that what I have heard is very slow and droning, but you'll find that most pro bands basically use their stage amps for tone and stage volume and let their sound system do the heavy lifting.
