Will a Rogue Audio Sphinx v3 perform decently with my Sonus Faber?

You should do more than ok with the Rogue Sphinx.   Used it in a room roughly your size.  

It plays loud, with oomph.  Very musical sounding.  Doesn't sound like a Class D amp.  

The phono preamp is also fairly competent.

Preferred it to the Rega Brio.  

One odd thing can be with the choice of speaker cables.  It will not play (at all) with Kimber's PBJ speaker cables.  Experienced it, thought I was going mental,  and spoke with the owner about it.  He confirmed it.  One of those things that had to do with the cable connectors.  Not making this up.  

If you are up to it, you can call Rogue direct.  Very welcoming bunch.  The owner will discuss things at length.

Rich 

going from incredible boXem to a middling tube amp doesn't make sense imo. get a solid stage integrated. the A-50 from Onkyo fits the bill and is a much more potent device as a whole with DIRAC DLBC

It will do nicely.  Good choice!

I have known this integrated quite well for the past five years with speakers even less sensitive (Magnepan and KEF) than yours and a bigger room.  I have also auditioned the Lab12 pre just a few months ago in dealer’s listening room. 

In a room your size you should have plenty of power to drive your SF’s to sane listening levels.  I typically play music at 75-80 db’s (A-weighted) and this was absolutely no problem for the Sphinx v3 (non-Magnum).  On rare occasions if I were listening to something like Tool and pushed it up to 80-84 db’s with the Sphinx, it would handle it quite nicely yet  85-87 db’s (again A-weighted), the soundstage would be effected negatively in my fully carpeted room; more so with the Maggies than the KEF’s, yet still noticeable. 

With a real solid DAC the soundstage at those more sane stated listening levels was really, really nice.  I had heard the Lab12 at a dealer and it too was a very nice performer.  I am going the other direction than yourself with a full separates (no phono needed) set up and I did not feel compelled to purchase the Lab12 as I was looking for a significant step up in musical reproduction.  As great as the Lab12 sounded, my thoughts were it would not provide enough in the upgrade path as I was seeking.  So yes, with a real good DAC you will get very close I believe to your current setup give the considerations noted, with the Sphinx v3.  It served me well for five years.  

I had the V1 of the Sphinx and found it a bit cool sounding, though SF speakers, might balance that out with their warmth. I suspect it has also been improved from V1 to V3. I have not heard the doubt that it will drive your speakers just fine, but I would also look around at other integrated amps. And keep the DAC separate.

Of brands I have experience with, I think the Musical Fidelity M6si would be good, and I am a super fan of Audionet, the SAM G2 might be a fit for you. Lots of used options on these.