+1
I think you should be looking at having your guitarist run that Sunn at 16 ohms to get a manageable and tasty sounding 90ish watts out of his head. I do that all the time with my friend's Sunn Concert head and Marshall 1960A 4x12. I run it on the speaker out of the head to the 16 ohm input of the cab. The mismatch cuts power, reduces headroom and adds crunch (compression) which will make your guitarist a lot quieter. Now you can go on and on about 150 watts vs 100 watts but there is an audible difference with these amps.
Also I might have missed it, but what kind of music are you playing OP? If your band is going in a certain direction, your guitarist's new Sunn amp might just be the first step in a series of steps each band member must take. And you or other members might simply actually be the stubborn ones that think your current gear and soft handed drumming is gonna cut it when it's not. Not saying that's the fact of the matter, but I've experienced this in situations with bands thsat I've played in where they're satisfied with what they have until after a few weeks of badgering them I provide them with my backline at a practice or gig and then they understand the wall of amps and vintage heads sounds better than their Peavey Transtube or undersized bass amp.
Just noticed in the quote of the post that appeared while I was typing mine that the OP's band plays heavy metal. Like Annihilation Time, Lecherous Gaze and Red Fang? (That's the vibe I get with the guitarist having a Sunn now) Then in that case the answer is more amp and more speaker because THAT'S PART OF THE STYLE!