You say 20watt but shoot for 18 watt, MAGIC number.
I cannot begin to list the players who have recorded classic albums with an 18 watt amp.
You have to register but 18watt.com has WEALTH of info. Can build your own or buy one VERY cheap imo.
Also, 18 watts is LOUD!!!!!!!!!!!!!
So, to NOT lose tone whilst pleasing neighbors think of power scaling!
Mojave have the Coyote which is uber cool 15 watter but costly which they say has power dampening. I am not 100% sure but am thinking thats their word for power scaling.
London Power has trademarked a box for power scaling (as refered to on Suhr's site in conjunction with the Badger, which btw is not all ptp "point to point" but some circuit board, not that it matters really).
And, power scaling is nothing like attenuation btw.
Dr.Z
Mojave
Reinhardt
65 Amps
Top Hat
the list goes on and on and on. And these are just the booteeky ones.
Also, maybe consider EL84's as they are in alot of 18 watters and give a real nice 'crunch' for that wattage/tone.
I could write enuff to send ya to sleep but am sure you will find a wealth of knowledge and help with a lil time.
Good luck :)