Thanks for the response!
I'm looking through those suggestions now to try and get a feel for them.
I looked them up briefly through mouser.com but unfortunately it seems they don't have any of the ADA4627. (Digikey has these, but they don't have any switchcraft mini plugs that I need.)
OPA627 also seems very expensive for some reason. There are also tons and tons of results, and while I can filter out the "wholesale" results pretty easily, I have no idea what any of the other results are, or why there are 5 versions of each opamp. The site might as well be in Greek at this point.
As far as the 2x1 adapter goes, is that the adapter i linked for "single" opamps, or is it something different?
Considering I have no experience with soldering, I want to make this as simple as possible (I don't want to ruin the M-stage accidentally.) I figured if I could decide on some opamps, I could just order one of CFG's adapters and simply plug n' play; assuming all the opamps were of the same type at least. If I end up with multiple types (how do you tell anyway?) I'll have to get both though, I'd assume.
My friend actually has a couple LM4562's, so I may give that a whirl as well, since it'd be the easiest.
Otherwise, I'll put the OPA627 and OPA1611 on my list. Perhaps ADA4627 as well, if I can figure out a plan to order from 2 places.
Are there any other suggestions? I also see an OPA637 mentioned; is this better than the 627, or are these just arbitrary numbers?
Again, these will all hopefully be class-a modded via CFG's adapters.
Also, the Shure SRH-840 and Audio Technica ATH-W1000X are both fairly easy to drive, while the Senn HD-600's require about 10 more gain to reach similar volume levels.
This basically means I'm running the M-stage at the 0 gain setting (I could switch it back to 10, but I'd lose some range on the volume pot) and I'm not sure if this would affect the choice of opamps, since I also recall reading some lengthy posts about opamps requiring specific voltages and gain and such.