Use the FX loop
I'm not sure if this works for all rectos, mine has a series mod FX loop so maybe it's just me, and this is probably not news to anyone but I found it to be pretty awesome. I run the 4 cable method with my HD500, and I'm able to get crunchy, badass sounds at relatively low volumes. We're not talking bedroom levels, but I can play guitar at 2 am without waking the neighborhood. I run the FX send volume at about half from the head, then adjust the master volume on the HD500. Keeping the HD500 at about half volume and the Mesa output volume at around 9 o'clock, I'm getting quality sounds at half the volume I'd normally need. Any lower than 9 o'clock on the head's output knob though, and it begins to sound bad.
Is this as good as a hotplate? No. Those things are outstanding. But running the FX loop and probably even a volume pedal you could accomplish some basic volume reduction without degrading the tone much at all.
Oh, and running Vintage instead of Modern late at night helps too. Yeah it's not the same sound, but this mode IMO turns the recto into a practice amp and it's nice at low volumes.
The problem with this approach is it means you're not running the power tubes as hard, but if you're not doing it for extra power amp saturation and you just want to knock it back a few db, I could see that working nicely.