RELs are a safe choice

any particular model based on current set up

Subwoofer / Low Frequency Optimization: See here

Mike

Hi OP

I don't own Sonus Faber, but I heard them play. IMO, they seem to go well with MC amps, and what herd was driven by Tubes. IMO, they seem to have synergy. 

So, in my opinion, the MACs are a good bet.  For Subs. I recommend that you look into REL, 2 x is possible

What sub do you have?  If you haven’t optimized both sub placement and settings I’d start there (I can provide a couple helpful links if interested) as that alone can make a big difference if it’s not set up properly now.  Likewise room acoustics can be very important especially if your listening position might be a bass null.  Moving the listening chair a a foot or two closer or further away to see if bass response changes significantly can be a quick/easy way to see if that might be the case.  My guess is that just dialing in the sub properly might go a long way in making a meaningful improvement if you haven’t done this already.  Hope this helps.