I'm pretty curious to know more about S1 vs M11/M11 Pro sound comparison too...
Doboo57,
If this helps, I was using almost exclusively the FiiO Q5s AMP/DAC with the AM3D THX AAA Amp, which is close or the same as the M11 Pro
( Sorry if this doesn't help since it isn't a Direct M11/M11Pro comparison, but from all of my previous auditions of both the M11/M11 Pro, the Q5s with the AM3D amp was the same as the M11 Pro, and the Q5s with the AM3E amp was a little bit better than the M11 but close. Again all IMO and YMMV. )
I've pretty much moved to the S1 and enjoyed the sound so much, that I also purchased a Lotoo PAW 6000 DAP (having sworn off of Android based DAP's and using other sources with the Q5 previously and Q5s more recently) this is a big change for me.
I really enjoy the Lotoo Sound Signature with the IEM's I use.
While I really did enjoy the sound signature of the Q5s/AM3D THX AMP combo, My first listen to the S1 had me rediscovering familiar music tracks, and started the move away from my till now tried and trued Q5s!
The Lotoo sound is much more neutral/natural
The FiiO with the THX amp is a little more fun with the bass.
All IMO....
I really enjoy the clarity of the highs on both Lotoo Platforms. the mids just seem a little more natural, and the mid bass is nice and tight. I'm surprised I'm not missing the bigger thump on bass from the Q5s vs the S1, and in fact pretty blown away on the clarity and quantity of bass this little amp/DAC can provide.
Now my dilemma is keep or sell my Q5s DAC/AM3E/AM3D amps or just hang on to them because I "did" enjoy them before but find I'm no longer listening to them as enjoying the S1 and PAW6000 depending on what I'm doing and what I can take along with me.
Again just my opinion but hope it helps you out.