Default standard for speaker "sensitivity" measurement listings? Anechoic? In-room? Other?

There is no standard. The measurements are often more from the sales department than the engineering department. Just use as a rough guide. 

Speakers which play nice with tube amps

Tube power is more expensive. That is why horns were so common in the 1950s when tubes were King. So you’re looking for a speaker with sensitivity in the low 90s at the very least, higher if the room is bigger.

As tube amps get larger, often they do not sound as good. OTLs are an exception simply because they lack an output transformer. So a more efficient speaker is helpful.

Avoid 4 Ohm speakers, and speakers with dual woofers IF the woofer array is thus 4 Ohms. Tube amps are less efficient driving 4 Ohms; you can lose an octave of low end response from the output transformer simply by using the 4 Ohm tap (loaded at 4 Ohms). Bass response is often a casualty of higher powered tube amps! So really pay attention to this.

Its possible to put similar woofers in series, thus creating a 16 Ohm load without affecting how the woofers act in the cabinet otherwise. A 16 Ohm load in the bass is great for any tube amp- it will be lower distortion and will have more impact. So if you can find a 16 Ohm speaker so much the better; they are pretty rare.

If the speaker is higher impedance you’ll have far less issues with speaker cables. The lower the impedance of the speaker the more critical the speaker cables become, to the point that monoblocks are needed so as to keep the amps as close to the speaker as possible so shorter speaker cables can be used.

@atmasphere Thanks so much. Very helpful post!