I'm circling back, now that I've had the RS1x's for a few weeks now. I'm still feeling them out, but so far they're my favorite Grados, after comparing them to my 125e's and RS2e's. Other comparisons I made were with Audeze LCD2, etymotic ER 4SR/4XR and Sennheiser HD600's.
Thanks to everyone here for the advice on break in. Over time the bass did improve a lot and the upper mid peakiness chilled out, too. Bass seems to be a topic with these headphones - two things I noticed: These cans go deep, but there isn't a lot of what's there. On one hand, lower frequencies are nicely detailed without any bloat or overhang. I don't mind instruments like bass drums and acoustic bass on the lean side, but it's when something is missing altogether that I find disappointing. There are a couple instances where there were instruments playing that I've heard on other systems, but were not there or unnaturally low, with the RS1x.
IMO the RS1x sounds like a hi-end medium sized studio monitor in a well damped control room. In the end, they are a little bass shy, and there's still a little peak around 2k, which can get slightly shrill in some instances with vocals, piano or guitars. All headphones/systems have their shortcomings, so other than those two things, these are stellar cans that can really dig deep into a mix.