If an attenuator was needed, I'd opt for a THD Hot Plate, or a Weber Mass. They're more versatile and a BUTT load cheaper than the Marshall. An Attenuator can be used with a combo just like it would be used with a head and cab. Grant it, it would be much easier if the amps internal speakers were connected to the power amp via a 1/4" jack, but most combo amps are like that anyway. And, if they weren't, it wouldn't be too hard to make it work.

But, there's not much point in using an attenuator with a solid state amp, as they pretty much sound the same on 2 as they do on 8. If you do hear a difference at louder volumes with a solid state, it's because of the Fletcher Munson curve (do a search and you'll find plenty of info on this).

Some more info on tube vs. solid state... One of the reasons tube amps sound louder than solid state amps, and better when pushed, is because tubes cover more frequencies in a certain frequency range than transistors do. Thus being percieved as louder and fuller/warmer (not steril) sounding. Also when a tube amp is pushed hard enough for the tubes to clip (causing an overdriven/distorted sound) the output is also compressed, which helps even things out.

Ok, I'll stop trying to be Lats now (who I probably learned most of that from).