A list of efficient speakers at ~$5K a pair.

My thinking is that Sensitivity + Impedance + Phase angle = Efficiency.  My understanding of these factors are not complete.

This is correct. However, there do exist some high sensitivity speakers that have rather tough impedance phase angles. Also, keep in mind that there is no real standardization for how manufacturers can rate their sensitivity. Some brands claim to base their spec on anechoic performance, while others include boundary/wall gain. Some manufacturers are altogether dishonest about their products’ sensitivity and/or nominal impedance. It’s not uncommon for brands to inflate the sensitivity spec by 5+ dB. 

What is your ultimate goal? To use flea-watt tube amps? Better dynamics at low SPLs? 
 

Given your room size, you are likely using less than 10 watts of power most of the time. This isn’t to say you wouldn’t benefit from speakers of higher efficiency, but it really depends on your goals. An efficient speaker is not necessarily a dynamic speaker either. Yes, higher efficiency does tend to correlate with better dynamics, but correlation ≠ causation. Some of the best low-volume dynamics I’ve achieved were from speakers of <85dB sensitivity. 

Much of a speaker’s perceived dynamic range comes from its bass performance, and thereby, its woofers. In my experience, dynamic sounding woofers have high compliance suspensions, low-mass diaphragms, and a low nominal impedance, preferably all three. It’s for that reason that the DIY speaker community tends to prefer 4-ohm woofers, vs the 8-ohm version of the same woofer. But it’s also possible to make such woofers sound constrained and lifeless with poor implementation. 

All this is to say that you can’t really judge a speaker by manufacturer specs. Even Klippel measurements are mostly useless for insight into subjective performance with your system, in your room. 

Fyne Audio Classic 8 Bookshelf should be on your list to hear.

I am searching for speakers that would make a lesser demand on a power amplifier than that of my Fritz speakers, which I do consider an easy load.

Just out of curiosity, why?  Are you sensing your amp is straining or do you think the speakers aren’t fully opening up?  Your room is small the Fritz speakers aren’t a tough load so I’m wondering.  What amp are you using?

These open ended questions will get you the answers you are receiving. What are you using to drive your speakers? I run 93db MHorn Mummy’s with a 12w Tron 211 amp and 17w Miyajima OTL’s in my room which is about the same size as yours and have plenty of dynamics and headroom. I couldn’t use a 2a3 amp but a 300b at 8w would be plenty on these speakers.  If you like your speakers now look at upgrading your amp.

OP

From what I gather, you seem to avoid speakers with low bass. 

in feb nobody seemed to be able to help re: Bel Canto REF 500s amplifier, what did you end up doing for amp power? 

"I am using a pair of Magnapan LRS+ speakers in a ~15x12x8 sized room and am looking for a SS amplifier to pair with my Aric Audio Super tube preamp. "

now

"I currently own Fritz Carbon 7s SE  MK2." 

...............................................

I am thinking more info will give members insight, perhaps post some photos in a Virtual System, it's pretty easy.

My take, you can put larger speakers with low bass in that size room, I would try that before mixing subs later to solve the lack of low bass. I'm not talking huge or extended low bass, not swarm, just decent full range capability with excellent imaging. 

I typically advise high efficiency so that it is easier to use Tube amps now or in the future, and avoiding low impedance as you intend is also wise for that possibility. 

Less Cost, Heat; More Options in lower price category, Easier to Place, perhaps Mono Blocks to disburse the heat and split the weight of big transformers.