It has a special circuit ("Dyna-Watt") which gives it noticeably more clean headroom than other 22-watt amps... which is why they call it a 30, although it isn't.
(It's not the same as a real 30W RMS amp either, but that doesn't sound like it will be a problem for you.)
BTW, Class A amps are not inherently louder. Class AB cathode-biased amps can sound louder than Class AB fixed bias amps or Class A amps though, for the same sort of reason as the Mesa Dyna-Watt works, by allowing the amp to run more 'wide-open' at the start of a note, which is where most of the perception of loudness comes from.
Yeah, I'm a little confused by your question - 20-30w is quite loud...
The only amp in that wattage range that I have first hand knowledge of a is '60s Ampeg Reverberocket - the 7591A power tubes stay quite clean until 7-8 on the volume comtrol. The Geminis are even cleaner...
If you want 'The King Of Clean' - up your criteria to 45w and get a Super Reverb.
To help clarify my question. I'm having so many volume issues when I play out I've had to move to smaller amps. The problem is I can't get the clean sound out of them I'm looking for. I'm also a humbucker guy which doesn't help much. I guess I might have to go with a channel switcher like a Fargen. I don't like attenuators and prefer not to have to depend on pedals for all my gain which I don't use a lot of. I have a Z-28 1x12 right now which smokes for drive, but I still can't get the cleans I want. I'm experimenting with speakers right now and maybe just haven't found the right one. Or maybe I'll have to use more than a 1x12.
I've wondered, too, if there's a way to design an amp that can maximize clean headroom through all stages and then overdrive all stages easily as the next level of drive/volume is required. This might allow an ultra-responsive reaction to guitar level as the amp hits its power ceiling and kind of function as an input responsive channel switcher. You'd still be stuck with having to run it to max ouput for best effect, however.
Maybe some amps are already doing this a bit?
I'm not shure if its to much power for you but the Mesa Lonestar Classic is a good amp to do all of the above.
I have the 100 watt 1x12 wide combo. It lets you go from 100 Watts Solid State rectifier in channel one then when you footswitch you can set channel two to go to 50 watts with Tube rectifier and it has a really good sounding Master Volume.
The Lonestar Special does the same with less power and more wattage options but its four EL84's.
You then also have the option of a footswitchable Master Volume or you can totally disengage both Master Volumes from the circuit with a hard bypass. With the footswitchable Master Volume engaged you can have four volume levels with only a two button footswitch. You can set channel one then step on the solo boost (second settable Master Volume) then when you go to channel two you have the option of boosting that channel for a total of 4 sounds. Channel two also has a defeatable drive circuit that gives you high gain if you want it. It uses half an additional 12ax7 tube but can be switched off which makes channel two identacle to channel one.
Then you have the option of choosing which channel you want reverb in and its footswichable too but takes an additional footswitch.
Lets face it its a Swiss Army Amp in a nice small package.
The Ampeg Reverb-e-Rocket II is about 22W and it stays quite clean with humbuckers up to about 12:30/1:00 where you start getting edge of breakup, a bit further with single coils. The cleans are positively gorgeous. This is actually notably louder than I can play at most gigs (classic rock stuff from the Beatles to Kansas/Boston/Journey/Deep Purple) and I wind up attenuating it a bit so I can turn up a little more. Anyway, one of the things I love about it is how loud and clean it gets.