System assembled; only a final decision about speakers remains

With regards to the OP:
Since you are using tubes, its in your best interest to get a speaker that is at least 8 ohms (especially in the bass region) and easy to drive (no difficult phase angles, higher efficiency well into the 90s at least). Tube power is expensive, and to get the most out of it its a simple fact that high impedance speakers allow the amplifier considerably better performance.

In the case of amplifiers that use output transformers, the lower the impedance of the speaker, the less efficient the transformer becomes. This can actually result in a loss of bandwidth on the bottom end- in some cases by as much as an octave! You'll also find that the transformer runs cooler when driving a higher impedance load; the heat comes from the power made by the output tubes turned to heat. Its better when its turned into sound by the speaker :)

An often over-looked issue is the speaker cable- with higher impedance loudspeakers (such as 16 ohms) the speaker cable is relatively non-critical; with low impedance speakers the speaker cable is really important as is its length.

@atmasphere Thanks for returning me to the original (and most important) unanswered question.
There are some basic facts I know to pay attention to with my amp (e.g. it’s monoblock tubes, w/ damping factor is 20, putting out 60 watts), etc.
When I look at potential speakers, some facts are given — their sensitivity, nominal impedance.
I realize, too, that different speakers have different power requirements at higher and lower frequencies, and that often one needs to know the nominal and minimum impedance — or, better, the impedance curve of a speaker.

I have read a lot of articles and even tried a spreadsheet as shared by Hans at this video:
www dot youtube dot com/watch?v=itKqSWH07_Y

What would help me is to have some basic information as to what a speaker’s specs should look like in order for an tube amp of X power to drive it comfortably.

Something like:
Assuming these specific variables (about listening distance, loudness, music type, room size, etc.)…
IF: you have a 60w tube amp with a damping factor of 20,
THEN: make sure you get a speaker that is, at least:
___ sensitivity
___ ohms nominal
___ ohms curve that looks like [insert description]
___ other metric(s)?

Having a rough and ready chart like this could help me rule out of bounds any number of speakers.

The Salk SS6m speakers I ordered mention/show their "ruler flat response" and note that they are "phase coherent in the crossover region." Then, the site gives the usual (pretty vague) specs as

• Sensitivity 90 db
• Impedance 8 ohms
• Amplification 50 watts

www.salksound.com/model.php?model=SS+6M

Jim S. told me that his speakers are driven comfortably with even 30 wpc in a room larger than mine. So that put me at ease. But if I entertain other speakers, it would be great to have a chart to suss them out.

need to ask jim what makes his Speakers better! he claims perfect flat response on website but there was never any evidence that is needed for perfect sound!! 

you also need to demand EVIDENCE of this perfect phase coherence in the crossover region. 

I wouldn't plug a power conditioner into a power strip.