My housemate uses one paired with a Lab Series head in a punk band and it sounds great, though it doesn't seem to take much more power than that head can give to it (around 200 watts I believe) as it tends to fart out a bit at higher volumes if he uses his bassier Rickenbacker with it instead of his Danelectro (though this is louder than he generally ever uses). Really most of these old cabs were pretty limited power-wise, so you'd probably have the same issue with a Sunn without some newer speakers added.
I have not ever played through it myself, so I can't give a direct comparison to the old Sunn 215s...
