I posted this on the TAG the other day...
Ok here's my take on C-tone offering...
Overtone Special is Skyliner hi-plate nonHRM
don't know why this amp is considered as #124. I don't see it this way, like I said OTS is hi-plate amp and #124 has fender style values in there.
Very smooth and warm sounding OD, lots of OD on tap but not as much as S&M, mid heavy cleans (in a good way). Amp that likes to sustain and compress. Easy amp to play... All knobs at noon and you have a sound to work with. I think this one is the warmest amp compared to others.
Overtone S&M (Sami and Marin) Special Skyliner nonHRM
Overall brighter amp compared to standard OTS... clean tone is more or less the same, on the other hand OD is brighter, more aggressive sounding. Like OTS this amp likes to sustain. In my experience this amp is not designed to switch from clean to OD on the fly. You have to set the clean and stick with it... than go to OD, set the tone again and play that for a while.
BTW this was the first D-style amp from Nik and it was his take on D-stayle amp... some said, it's to harsh sounding but some others liked. Sami and Marin didn't want to mod it (like OTS "jzucker" mods) and that's way it's called S&M model. I don't blame them they sound very good with this amp.
Now I don't have any experience with FM or 183 models... but looking at layouts, I can say that FM is as well as OTS hi-plate amp but has some things in there which will make the amp sound brighter (master cap, presence cap, smaller snubbers)... so I guess this amp is designed to be used with D-lator... without it it might be just to bright.
183 is again hi-plate amp but with some interesting turns... like very small subbers, no LNFB, different style PS. I really don't know what to say about this amp... but you heard TAGs Politician, what's more to say.
Like I said this is my take on these amp... I played the OTS and S&M and I hope I'll try the other two as well some day.
...but you all know this, tone is a personal thing and what is smooth and warm for me, it might be dark and dull for somebody else.