Choice of amplifier

If they can bridge mono, a pair of those would be really tough to beat!

Gavetti, it appears that the Extrema may have a variable impedance that dips as low as 4 Ohms, with a nominal value that is 5 Ohms if I recall correctly. This suggests that an amplifier with a high output damping factor may yield very good results, mayhaps better than just a high powered amp. You may want to give a try to the Bel canto Ref 500M monoblock, if 250W per channel into 4 Ohms doubling into 4 Ohms were adequate, or its bigger brother, the Ref 1000M monoblock, with 500W per channel at 8 Ohms, doubling into 4. I have used the exceptional 1000M very successfully with the Vienna Mahlers V1.5, which are also difficult speakers to drive. See my 1000 Mk.2 (recently renamed 1000M) review on PFO at:
http://www.positive-feedback.com/Issue43/bel_canto_ref1000.htm

An Ear890 running into mono might not make it with the low impedence characteristic's of your Sonus.Here is a link from Stereophile about your speakers.More like a 5 ohm speaker.I would consider something that could drive 4 ohms all day long.When you run a lot of amps in mono,8 ohms may be the lowest for them.EAR doesn't seem to give any spec's on the net.Link>>[http://stereophile.com/standloudspeakers/915/index3.html]

Of course if you want to go class D, which is not a bad thing at all, few amps can keep up with the Wyred4Sound amps for overall performance and bang-for-the-buck. I have their STI-500 in my other system and it is very good indeed. Dynamic capability is among the amp's strong points and it puts out a healthy 550W/channel into 4 ohms. If you have any specific questions about its performance you may e-mail me privately.

My cent .. Gavetti

I suggest you the Parasound Halo JC1's on the Extrema
I know very well the S.F Extrema sound and I'm sure the sweetness combined to the brutal power of these monoblocks are a perfect "Matrimonio" (marriage)
I have them on my Thiel 2.4
Great loudspeakers too

Curio

Merry Christmas to everyOne