Just received my c421. Kudos to John for the quick shipping!
The outer case has scratches and scuffs (as we were notified about before ordering), but they're not as bad as I was expecting. The faceplates don't quite line up 100%, and a couple of the screws don't really sit flush with the faceplate, but it doesn't really bother me.
I like the feel of the switches. Sturdy, but not too stiff. The volume wheel is easy to move, but not too loose. To me, the feel of the switches and volume wheel are perfect.
I brought a variety of IEM's with me to work today for some initial listening. Unfortunately for me, my UE4's are unusable with the c421. At listening volumes and low gain on the amp, I get a channel imbalance. It goes away if I turn the volume up, but then it's too loud. I was in a similar situation with the Practical Devices XM6; however, the XM6 had a 75ohm impedance switch. My Headstage Arrow didn't have this issue, either... perhaps that might be a nice addition to the c421 if John decides to make a newer version or something. I wish there was a gain setting lower than the "low" setting on the c421 for my more sensitive IEM's.
As far as the bass boost goes, it wasn't quite what i was expecting. I was expecting it to add a tight, firm, punch, which it does; however, it adds a little more than that. I can't quite pin it down, but it possibly adds a little bit of warmth... almost tubey sounding? I'll listen more and figure it out tonight.
Anyhoo, I'll do more listening later when it quiets down at work. I've brought a little variety of IEM's with me: Etymotic ER4S, Ultimate Ears UE4 Pro, Ultimate Ears TF10, Westone 4. I shall report back later!