Speakers for 2nd System!

Compact and efficient are pretty much mutually exclusive.  The closest thing that comes to mind would be an offering from Omega:

https://omegaloudspeakers.com/collections

Your amp has more than enough power for a more typical 90-91dB speaker in that size space which would open up your options considerably.  Unless, you need to listen at 95+dB levels, you should be fine with a more realistically spec'd speaker. 

He forgot to mention " The speaker should also make me soup & sandwiches" on his want list.

your going to have a VERY hard time finding everything on your want list..

This is a great project. Look at the Devore oBaby, above Audio Note, you can chase those way over budget if you want, Fyne makes a small speaker but not sure of the power needed. Good luck and please keep us in the loop as you put this together.

Good recommendations so far, keep ‘em coming! 

Omega and Horning are on top of my list. 

I do not want speakers in 86-90dB range cause I have plans to rotate SET (2W-8W) amp / pre combo down the road. 
 

I will speak to brands I know, but not knowing your music preferences, understand that my listening is jazz and classical and I value clarity, accurate instrument timbres and tight bass.

+1 for Totem - I own a pair of 8 yr. old Sky Towers, smallish, but with surprisingly deep bass. Totem makes several  bookshelf models, all in the 87 dB efficiency range, a couple that will not require more than 20-25 watts per ch in a small room. Bass down to about 50 Hz on several, and lower for models with larger woofers. One advantage is the ability to place these speakers within 6" of a rear wall. They mate well with tube amps.

+ for ATC - The SCM 7 is remarkably clear and coherent, but needs a sub given the small size of its bass driver. Skip the SCM 11 and try listening to the SCM 19, but your amp will probably not have the power to make it sing.

Aerial Acoustics 5T - about the size of the ATC SCM 19, but a bit more efficient at 87 dB. Will sound great with your amp if you’re not requiring very high volumes. It’s a 4 ohm speaker. Again, lots of detail and coherence with bass down to 50 Hz. Also  can be placed close to a rear wall on shelves, in bookcase or on stands.

I prefer all of these to the Harbeths, Spendors and similar BBC monitors, especially with a tube amp like yours, which will provide a rounder, fuller sound. All of these speakers excel at detail without being tiring. But it’s all a matter of taste.

Good luck in your search.