Some things come to mind:
- Be careful with smaller tube amps like the 20/20 when used for this purpose because they will distort when pushed hard, Mesa markets that as a feature. A 20 watt tube power-amp is equivalent to a Fender Deluxe, and sounds great when pushed, and won't be unbearably loud, but we want that distortion.
- Reducing the output of a 20 watt amp to 10 watts will help clear it up, but the available clean volume will drop by 6db. That's pretty apparent.
- Modelers need extremely clean power-amps if you want the modeler to do what it's good at. Of course you could turn off power-amp modeling and rely on the tube power-amp to supply that backend distortion, but it won't change to sound like the original amp when you switch models, only that model's preamp into your given tube power-amp. And, if you're only using the power-amp with your stage monitors, the sound you send to FOH will not be the same.
- The sound of a distorting tube power-amp into stage monitors isn't very pleasant because stage monitors have a lot higher frequency response than regular guitar cabinets. I'd use a pair of small 10" or 12" cabs loaded with good guitar speakers instead.
50 watts is only 3db-ish louder than 20 watts when running clean. That's the difference between our normal cruising volume and when we step on a solo boost; It's not very much. Adding wattage for a tube amp means adding a lot more weight and size for the transformers and capacitors,
and tube cost is going up fast, something to be aware of these days. That last point makes me really happy I have my Fractals because I've sidestepped that rapid tube-cost increase.
The 5050 bridged in mono would be good, or the LXII in stereo. I wouldn't go with anything less than 100 watts if I wanted clean reproduction on stage that matches FOH.
My Imperial combo is 20 watts when wide open, which is how I use it, and I stand about 3-4 feet in front of it with it pointed at my head so it acts as my own monitor and room sound. It's comfortably loud but the power-amp is definitely overdriving at that point, which is why I'd hesitate to use 50 watts for reproducing a modeler's output cleanly. 50 watts will only improve clarity a little before distorting. 100 watts will add 6db which would be a comfortable cushion I think.