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.