I took a sound pressure level meter and measured the Collings by holding an E major cord and strumming like crazy, with the sensor mic about 6 inches from the sound hole. Peaked at 106db. My Martin D28 (dreadnought) peaked at 105. The two sound good together - the Martin has a slightly fuller spectrum, mainly in the lows, and the Collings cuts through very nicely.
Yeah, Greg, that's the headless cabinet for now - I was getting squidgy volume drop-outs (followed by soul-crushing Silence) from the JCM800 head, even after replacing all the tubes - it's in the shop, and I feel like a have a close relative in ICU. Hopefully it'll work out. I bought that rig in 1986, and I've been lugging it around ever since. The SG is much newer.