In tube systems is 90db efficency really the low?

It also depends on the speakers' impedance curves and complexity of their crossover networks. Speakers with benign impedance curves (no dips much below 8 ohms) and simple crossovers can sound great even with low-power amps, even if their sensitivity is in the high 80s. Example: ProAc Response 2 speakers, which I've head sounding fantastic powered by 6 wpc SET amps.

I use 90 as a bare minimum. Yes, my 17 watt triode amp has run maggies, but was lacking. Simply put, the higher the rating the better if you only have a couple of watts.
Keep this in mind:
90db at 1 watt
93db 2watts
96db 4 watts
99db 8 watts
102db 16 watts
Every time you double the wattage you will see a 3db increase. To double the volume, you need a 10db increase. So, 92db speakers should require 1/2 the power as 89db speakers. 96 db speakers(lowther) will be twice as loud as 86db speakers(maggies).

And it depends on what loud is to you. It takes some feedback with your dealer...one that knows his stuff.

The Superamp from AES (a division of Cary Audio) is 15w/channel and designed/voiced with Maggie 1.6 (86dB)

Small room? No problem. Big room? Even with big power those speakers won't move much air.

Push-pull 15w amps sound ballsier than 15w SE amps to me. But I would have to bench both to see who's fudging.

Watts are watts...or at least they are supposed to be.

Also...one last thing. Some 90dB speakers I know I would not run on even moderate power SS or tube amps. Sensitivity tells you something...but not everything.

My tube system is driving 80db speakers
It's how you match the system.
My amp is a 55Watt/ch push and pull stereo tube amp
i.e. if you have a 89db, you should have at least a 20Watt push pull tube amp although 15 Watt does makes sound but lack of control in bass. Damping factor on your speaker also plays a role.