What do you think of a Densen-Concertino pairing?

Have you tried the Densen web site ? There's a chatboard where people discuss their systems.
I own a Densen B100 and it's a great amp, but as you say, it has a reputation for being a bit fussy. I have only heard the SF concerto, and not with the Densen (the amp was Quad) and I remember them not being bright, and having good bass for their size. I think the Densen can be bright with the wrong speakers, particularly metal tweeters, so I'd guess it could work quite well with your SF concertinos.

If you live in SF bay area you could bring your speakers over and try them on my amp if you're interested.

I had the Beat 100 paired with ProAc Response 1SCs. I would believe the similarlity between with ProAcs and SF should result in a fine system. I auditioned the CJ CAV 50 and Mesa Tigris before settling on the Densen. I liked the Tigris more but, only marginally and the Beat was a much better value.