For serious work I will pick up my 100% factory stock PS-550 over my stock "Carbon Edition" & MMWS PS-6100 any day of the year.
The PS-550 is so well mannered yet grunty with a semi short 3/8" 56DL B&C setup she doesn't even wear me down through hours of bucking up logs.
The issue with the PS-6100 series saws is they utilize the exact same clutch assembly as the PS-420/5105/550 and it is just too lightweight/weak to keep up with the power of 61cc's of power both stock and even more so ported.
The PS-6100 is a nice design, but it really needs the heavy duty clutch of the 64/73/79xx series saws!
Even a stock PS-550 will make her clutch slip when biting down into hardwoods with a fresh well "to specs sharpened" quality chain!
The PS-550 is as underrated as the the PS-6400 - then again, maybe I just have a sweet spot for the under dogs!
For anything under 24" my PS-550 & PS-6400 are my main go-to-saws - period!!!
P.S.:
There is nothing wrong with my MMWS PS-6100, nor my XS EA5000 for that matter!
They are simply so temperamental and excruciatingly tiring and exhausting to use for hours on end that they are not my first pick for "work" (my last log pile bucking took 4/2/4 hours).
They are both brutal, no questions asked!!!
Very fun saws for "play & show off" - but also, I don't want to wear them out on crappy jobs.