Silverline or Sonus Faber

I have never heard the Silverline's, but have had many SF models and I have listened integrated amp that you have.
In fact, I even had the Pathos amp as well, but it was Inpol 2 model.
If your space and budget allows you, my advice would be to look for 'better' speakers (still SF, of course) as your amp is capable for it.
Perhaps SF Cremona Auditor M, if you dont mind buying used ones.
I have listened (had those speakers as well) that combination and its something that could keep you satisfied for a long time.
As for second question, generally speaking, its better that speakers have higher impedance as they present easier load for the amp.
But, with your amp and almost all SF monitors in a room that is not too big you should be fine 

Beautiful amp. Congrats! I’m not a big fan of SF’s “cheap” line. I say cheap rather than budget because I demoed the Venere 2.5s in my system and they felt cheaply made — because they were — and sounded relatively unrefined. These “cheap” models bear no sonic resemblance to SF’s pricier upper lines BTW, and I doubt you’ll warm up to them given you’re coming from the excellent Sonatinas. IMHO Chameleons are designed to look cool and be affordable to the younger generation, just like the Venere was. If you want an indication, despite being 2”-3” smaller than the Chameleons in every dimension, the Minuets actually weigh 25% more. The Veneres were likewise lightweights and the 2.5’s cabinet sang like a jailbird to the extent that it intruded into the music with a measurable upper-bass hump that was impossible to tame in my room — the only speaker that’s been able to pull that off. Obviously I’d steer you toward the Minuets, which I’ve also heard and would own in a heartbeat. And unlike the “cheap” SFs, the Minuets do reflect Alan Yun’s design skill. Hope this helps, and best of luck.

I have experienced both and in fact purchased the Silverline MS for my son.  Like SOIX I am not a fan of the lower level SF speakers.  I did own SF Amati Tradition Homage speakers and they were a great speaker but obviously a totally different animal.  Totem Speakers might be an option also.  

I use the Mk3 model of the Pathos with my 4 ohm Spatial Mk3 Turbo S's. The Pathos handles 4 ohms no problem.