Blackstar Artist 15 - 15 watts, 6L6 tubes. Channel 1 bypasses the preamp and goes straight to power amp. Channel two has more tone shaping options and can be used for anything from clean to classic rock crunch.
Spectacular pedal platform but can stand on its own as long as you aren’t aiming for high gain.
The usual definition of wattage for an amp is the highest output an amp can make while staying below a defined percentage of harmonic distortion. So, you’re basically asking which company fudges with that figure the most?
That being said... just get a Morgan PR12 and find the most efficient speaker you can find to put in it. That matters much more than whatever you’re thinking matters here.
Princeton. Non-reverb. Blackface/silver face whatever (pretty sure [though I could be wrong] that the circuit remained the same). It won’t break up with single coils until well up the dial. A sleeper amp for something similar is a Bassman 20. Rivera era and has a healthy dose of mids if you want them. The Fender Musicmaster bass amp has decent headroom if you single coil too.
I have an Allen Sweet Spot - 18 Watts with JJ 6v6 power tubes. Speaker is a Emi Li'l Texas neo that brings the weight down to 28 lbs. If you go with the 6L6 option, it's rating is 25 Watts, so you should have good headroom.
Additional recommendations would be Dr. Z Jetta (30 Watts, 34 lbs* or Cure (15 Watts, 33 lbs*). "Z-Watts" are louder than you would think.
* Using a neo 12" speaker will reduce the weight by about 4 lbs vs the stock ceramic speaker.
Light weight and lots of headroom don't really go together in a tube amp.
For big headroom you need big iron, and that adds weight.
IMO if you're looking for clean headroom you're going to need more than 20 Watts.
Couple of candidates: the Mesa TA15, now discontinued so you'll have to find it used, but ch1 the Vox channel or ch2 in Tweed mode both get hella loud with high headroom when I pair it with a 27" wide Mesa 1X12 open back cab.
the second contender is a VHT Special Special 12/20 - designed to be a loud, clean headroom pedal platform from the get-go and it delivers. It comes with 2X6V6 for the 12 watt power, but a little internal bias switch lets you run 2X6L6 for twenty watts - It's voiced like a Princeton Reverb with just a little more mids than the typical scooped BF-Princeton (the cab is oversized compared to a PR, muxh deeper, so it is not boxy sounding). Handwired for easy maintenance - great value in a good sounding amp. Good Hunting.