Are you looking to buy one strictly to make the recording silent?
As long as you have a cabinet hooked up to the amp, you wouldn't need a power soak. This would mean, of course, that for some amps you'd have to have a space you could have the cabinet blaring.
I used my XXX on the clip I posted, and I had the channel volume on the Crunch channel all the way up, but my master volume was all the way down. I could just barely hear the sound from the cab, it was definitely under "TV volume" levels. I would imagine you could do pretty much the same with any master volume amp.
For the 6505/5150, though, I know I'll have to have the post gain up a bit to get the signal level high enough to work since they don't have a master volume.