Belles upgrade??

Currently own Belles 350A Ref, original version. Review of 150A Refs in mono sounded intriguing. Wondering if those or MB200 monos would be any kind of upgrade? Modwright pre, SF Liuto speakers...listen to a lot of different stuff. Thoughts? Opinions?   Thanks in advance. 
Never heard the Belles amp, though I have read the Pass Labs amps are on another level. Currently own a Pass X150 one of the older ones, but see no reason to upgrade with my Supratek preamp.
I had a Modwright preamp before the Supratek, comparable with a different flavor. I would think the Modwright would pair well with Pass
go for the .5 or .8 series which are suppose to be a little warmer.
Hi - Thanks for the suggestion, will consider. One of the biggest curiosities I have in this thread is whether I would get big gains in performance by going with similar monoblocks as opposed to stereo amp? Klaus from Odyssey states you get biggest performance improvement with his gear by going to monos versus upgrading his stereo amps.   
It's mainly in the power supply. Two separate and independent ones in monos generally perform better.
I tried a pair of MB200s for a month or so last year and it was "ok", not forward, not annoying but yet, rather musically boring.  Take a look at my system to get an idea of the speakers I have.  I tried the MB200s on both my speakers, which can be a bit demanding of amps for bass control.

A change to the entry level stereo amp, Symphonic Line RG-11, rated half the power of the MB200's and even though the RG-11 could not play quite as loud, it was far far more full and rich, 3D, etc.  Incidentally, I also tried the Odyssey Stratus and the RG-11 again was in a different league.  Moving up to the SL RG-1 stereo amp allowed my speakers to shine shine shine.  I dream to own the SL Kraft amp(s) one day if they take this performance to a new level as claimed.

If your budget is in the $2k or under range, consider other amps like the McCormack, BAT or even the Plinius 102.
Oh, I forgot to mention that Klaus is the USA SL dealer.  He might have an SL amp for you to try.  Once you do, you will not return it.
Again, thanks for all the info and suggestions, will research further. Soix, to answer your question, areas that I think I would like to see improvement are richness and dynamics, and also noise reduction. I know the tubed pre tends to bring in some noise on its own and I will be upgrading my wiring situation in the near future, which should also make a big improvement, but the review implied running monos was also a huge factor. Does anyone concur on that point? I ran an ARC 100.2 a little while ago, every other piece was the same. It seemed more dynamic and sounded pretty amazing, but it was a good bit noisier. 
Not sure going to monoblocks is going to do too much to help add richness.  There’s a nice McCormack DNA-2 on US Audiomart for $1750 that, if you want, you could send to Steve to do any level of his upgrades to make a very special amp.  I find McCormacks to have some richness and oomph in the lower mids that may fill in some of the richness you’re looking for.  Obviously dynamics are not an issue.  Best of luck. 

Belles are hard to beat they are that good, the money is put inside not outside like thread is today, and when you look inside no better than many at less than 1/2 the price. Overbuilt to justify the asking price.