I've read through all the suggestions here and the one thing I found interesting is that not one of you mentioned trying a 1x10 inch cab. My recent discovery, after being repeatedly asked to turn my amp down, (OTS 50watt w/ EVM12L 1x12 D-style), at church was that just by going to a 10 inch Jensen Jet was a very useable volume and still had all the tone. Built a D-style, closed back, ported cab for it too...sounds quite exquisite actually. My two cents. By the way I am the chief sound engineer at my church.

Check the master volume pot. At "1" the amp shouldn't overpower a bar.

I would recommend using the Dumbleator loop as your master volume control. IMO it sounds better than lowering the amps master.

The triode switch is not as bad as those on some amps, it does affect the tone, but not as much as using loads of FX
An attenuator will also suck some tone, but the THD ones are acceptable at -4dB (stills some tone lost), which is usually enough to make a noticeable difference

Have you seen the variable-efficency speakers?
Eminence do one now: http://www.eminence.com/fdm.asp

I use my 50w in a home studio, it's not too loud I think unless you run the master vol past 3 o clock, and keep the clean channel clean. I think the master vol and OD trim could tame it for a function room really, unless you use big OD from your FX

Yellowjackets change your 6L6 amp to a class A EL84 amp, not really advisable if you want to keep the same tone. ToneBone adapter is the same idea
6V6 will sound less crisp I think, and less bass snap.

a portable ISO cab would make it quiet, but then you would go through the PA, not ideal if you're a tone purist, although tbh if it's multi-style function music with multi FX, maybe that could work better once you work out how to get the right result.
only the bigger ISo cabs with 12 inch speakers sound complete.

cheers
Tone