Best Values In Speakers To Pair With Coincident Frankenstein MKII Monoblocks?

A good friend who I have gradually been introducing to quality audio for the past few years has decided to start by putting together a SET system, based on demo's, attending both Axpona and Rocky Mountain this year, with me. He started his new system by purchasing a mint condition pair of the superb Coincident Frankenstein MKII mono's, and we are now currently attempting to match these amps up with a suitable pair of, at least somewhat reasonably priced, suitable high efficiency  speakers.

His room is 13.5x15. We would appreciate any insight, recommendations and suggestions, from you, of speakers that would pair very well with the Frankenmsteins. His budget is $5k or under for a used pair (to maximize his budget). Any speakers within the budget, that you have heard working well with these amps, is fair game, excepting any speakers made by Coincident. I would appreciate it if those are excluded from the discussion. Most are too expensive anyway, and there are other reasons for the exclusion. Not that Coincidents aren't fine speakers, but they don't fit into our scenario. Thanks for any help with this, it is greatly appreciated.