Is there any pinging from bottoming out key strikes in the FC660M that might require
dampening? I've had to dampen my CMQFRs and Filco M2s, regardless of the MX switch.
If the board requires sound dampeners will depend on your personal preferences. I have an FC660M with browns, and I tested red o-rings of the softed ones, I think they are 40, and I do not like them. The main reason was that the board switches are plate mounted, and the rigidity of the plate makes the switches to feel very solid, and when bottom them the sound is in the lower pitch, wich is very nice. If you get one, you should try some o-rings just to see if you prefer yours with, or without them.
There are some videos of the keyboard in the internet you should check to get a more closer feel of how the board sounds. This
video is of a 660M with blues.
As others have already said, the construction is really solid, heavy for its size and the finishing of the plastic has a mate nice feeling, almost as if it were rubbered.
Note: If you are interested in replace the caps you should know that the windows key and the right shift are not standard. The first is 1u instead of the more common 1.25u ANSI, and the right shift is 2.25u, the same size of a left standard key in an ANSI keyboard. But the main issue for replacing the caps is the spacebar. It is 6.25u, which is standard, but the main difference is that its stems are not located in an standard position. Thus, if you want to replace the caps you should get the board in a color close to the set you are interested in to keep using the same space bar.
The FC660M is available in black, white (near cream), and in charcoal gray. This last one comes with dark gray caps, a little bit ligther than the keyboard housing. I got this last color, and I also buy a white, and a black space bars. The white one was sourced with QTan, and for the black I bought a black set for KC84 at Taobao for 14 USD that comes with an space bar that has the same stems positions than the FC660M.