Some of you may know my current project is an odd blend of Tweed Pro and Blackface Vibroverb circuits. (tweed pre, vibro power). I am looking to measure how much my amp is putting out, to help find a decent lower wattage 15" speaker that will work. Currently, with my 7581a tubes, 450v plates, and Mercury Magnetics upgrade Vibroverb Output transformer, I measure 30v out at the speaker with the amp on 10 and me hitting the guitar as hard as I can. Can anyone help? I'm not looking for a precise wattage number, Just a narrow range to know what I'm looking at for wattage. I'm going to be hopefully switching to 5881 tubes soon to drop the wattage even further, but this way I know the MAX my amp is sending to the speaker.