Musings, which may or may not be applicable.
Guitar speakers are almost always louder than bass speakers, except in the lowest register. It's basic acoustics: it takes more energy to move air at low frequencies than at higher frequencies, so guitar speakers, and many guitar amps, simply don't bother. In addition, human hearing is most sensitive around 1-2KHz, so a lot of guitar speakers are optimized to have their greatest output there. That makes them sound louder, even if they're not.
I'm not familiar with the stoner/doom genre, so I don't know what kind of sound you're going for. Maybe bass speakers are more likely to get you that sound, but I doubt it.
More speakers usually gets you more volume faster than more power. The silver face Concert amps were stable only down to 4 ohms. The black face models claimed stability down to 2 ohms, but I have read knowledgeable people saying that the circuit topology doesn't support that, so I wouldn't trust it at 2 ohms. So, if I wanted more volume, I'd try to use two 8 ohm cabinets or as many as four 16 ohm cabinets, depending on what I had available and could transport.