Attenuators are expensive and never really sound quite right for several different reasons.
FWIW, there are very few amps with master volumes I even like the sound of, much less ones with channel switching and multiple "gain" knobs. When I want to goose an amp a bit for more dirt, I usually use a
- Bad Bob - a somewhat gritty 'clean' boost
- ToneFactor Mule - a Red Llama clone that does nice 'edge of distortion' sounds without a midboost, as well as some funky Neil Youngish sounds when the gain goes above 12:00
- Fulltone OCD for higher gain sounds
All of those sound infinitely better, and more 'natural' than any amp I've used with preamp gain or channel switching.
The only amp I have with a master volume is a 4-input Marshall clone. I use the MV mainly for preamp voicing, and not for dirt especially. This is because it's got a big old 500pF bright cap across the volume control, and it'll take your head off if it isn't turned up to 5.