Reasonably sensitive speakers for tube amps, max $2k/pair, new or used


While this topic has been covered before, I'm interested in both recent experiences with new products or anyone who just would like to chime in.

I have a couple friends interested in tube amps, lower wattage -- 40 wpc or below. Before they pull the trigger there, they need to find the best higher sensitivity speakers they can, hopefully at $2k or below, new or used.

QUESTION: What high sensitivity speakers would you recommend to them, new or used? What did you try and NOT like?
Characterizations of their sound, experience with a trial period, the company, etc. appreciated.

I'll pass the best answers and links along to them.
128x128hilde45
@jjss49
Outstanding explanation about the pitfalls of efficient speakers. It’s great to have someone on this site that actually knows what He/She is talkin’ about!!
There will always be Exceptions to the rule. Case in point...
Emerald Physics Open Baffle speakers, specifically the 3.0 and up offerings
I have 3.4s which combine a 12" woofer with a 1" concentric polyester tweeter (~97dB), plus, a small amp friendly load. Amazing value for sure (see my Virtual System for picture). In most 'normal rooms' they will be all you need, but mine is far from that, so I supplement with 2 SVS powered subs
Monitor Audio Silver 300 - $2200/pair and less
Clean design, nice tech, bi-wire/bi-amp reedy. A lot to great reviews. 
@hilde45 

I've read through all the suggestions here and based on the fact that your friends have subs and are looking at "mid-powered" tubes I would have them talk to Fritz and see what they can work out.  He may in fact have some models that have been out for in home audition that he can work with them on. 
@jackd Thanks, good suggestion. Thanks to others, too! Monitors sound worth a try.