I'm thinking about a low watt tube amplifier?

Just a thought. A big part of tube sound is their low damping factor. You need way more power than you're using even at reasonable levels if you play music with wide dynamic range. It's only for micro seconds but pianos can give 30 dB peaks and that can upset amps even after the peak is gone. As a guess look into PASS amps. They have low damping factor like tube amps and they make class A like amps that are more powerful than their rating to handle large peaks. The class A amps are class A at their rated power but usually double and go into class B for peaks. But almost all the time you will remain in their class A rating and Pass amps have good power supplies and recover basically instantly from high peaks.

Don't do it! Maybe you haven't been reading the threads, but the consensus among the know-it-alls is more watts sound better. 

(I kid, but too many people believe such ignorance).

I believe for a 87 db efficiency you gonna want some more power.  Those just aren't going to wake up until you get some wattage running through them.  I do agree, more watts, is not better necessarily.  I run Decware, 6 watts/channel through Altec Lansing A-7's, 16 ohm.  Don't know the efficiency.  I have seen everything from 95 to 110.  It gets very loud and does everything I need it to do.  For years I ran some JBL 4312's with a Dynaco ST-70 and this was plenty of power.  I believe efficiency is 90 db.  At the end of the day, some leg work will save you grief.  Either get some loaner amps or load up the speakers and take them to the store.  Listening in your own space is the best but listening at the store, you can get some great generalities.  The Altecs are as big as a washer and dryer, you mentioned size restrictions, so probably not an option.  

I'd stick with the Cayin and Luxman amps. Adequate power - and plenty of tube "magic" - contrary to what some claim! Plus enough in reserve to handle music peaks on your 87db speakers!

I would agree with Roxy. Your speaker may be rated at 97db (I have read knowledgeable reviewers draw attention to the fact that manufacturers do sometimes exaggerate these numbers). The important question is what does the impedance look like? If you have 90db or above speaker with the right drivers, that is rated 8ohms or above, and keeps there mostly, perhaps only dipping close to 4ohm in a narrow frequency range, you maybe could get away with low power 300b or 2A3 poweramp for the right music. If you listen to a lot of heavy rock or techno type music (and i do because every genre has great music), you will experience low-powered amp reaching its limits quite regularly. A se triode 15 watt such as Art Audio Gill Signature will fair much better - quite surprisingly so. In the zone though, those 2A3 amps are mighty sweet and highly addictive, especially with vinyl.