Colossal volume! I saw them a couple of years ago and it was a unique concert experience - certainly the most visceral sound I have ever experienced, literally gut-shaking. There were flakes of plaster coming from the ceiling of the venue (SWG3 in Glasgow) and O'Malley played some palm-muted notes that actually sucked the air from my lungs. Make sure you wear ear protection - even with 20dB Etymotics in place I couldn't hear properly for 48 hours afterwards.
Oddly, despite the overwhelming volume, I found it quite relaxing - like a sound massage.
This is Sunn 0))) fan The Needle Drop's description of a Sunn 0))) show: