Midbass under 100$

Ok, thanks so what drivers would you choose with a limit of 600$

For 4/6 speakers (2/3 way)

If you want to challenge the resolution capabilities of something like a Sonus Faber, you may need to increase budget a bit, but it's hard to know for sure without trying them all.  As mentioned, Seas and Scanspeak are excellent,...maybe look into SB, Satori, or Morel too.  You may find some used stuff too.... Vifa, Focal, Audax, Dynaudio Peerless, etc.

I totally agree with knotscott.  The Concertino had excellent resolution in the mids and upper mids. It has a rich tangy sound to the midrange texture (it's hard to describe).  To reproduce this is going to be very hard.  The best I can come up with is to get a Scan-speak paper cone midwoofer such as the 18W/8542-10 7" ($213).  This will get you excellent resolution for the mids and upper mids.  It may not have that rich/tangy flavor of Sonus Fabre, but I think it's the closest you will get.  Then combine it with a Seas Excel T29CF-002 (E0040) Crescendo Fabric Dome Tweeter ($362).  This tweeter is has excellent resolution, but it is just a tiny bit forgiving because it's a fabric dome.  It is very sweet sounding and very forgiving (just like the Concertino soft-dome tweeter) and it has a sweet/colored sound that makes you think that you're listening to a tube amp. 

There may be other combinations, but these drivers are what I know.  I probably would not combine these two in my own speaker, but you're chasing the "sound" of the Concertino and I think this will get you somewhat close.  You can get these from Madisound and they will also design a crossover for these two drivers as well.