Here are my impression over a few opamps in the E12DIY. I stuck to BUF637 this time and will look into LME49600 some other time.
AD8620 - good detail, almost edgy, a little flat on soundstage and lean on lower end texture
MUSES01 - near perfect balance. ultra smooth and full sounding with lots of texture
MUSES02 - Better depth than muses01, more layback with a little less texture
MUSES8820 - similar to MUSES01, but scaled down and not nearly as grand
MUSES8920 - similar to MUSES02, but similarly scaled down like 8820
LME49720HA - very clean and clear with good air and soundstage
OPA2107 - a little mid centric, nothing remarkable
LM4562 - similar to LME49720, even cleaner and leaner, very O2 like.
OPA2209 - a little veil on the mid and lacking detail
AD8066 - great micro-detail and decent soundstage, but otherwise unremarkable
AD797 x2 - neutral but edgy, like AD8620 with better depth
OPA1611 x2 - laidback, if not a little dark
OPA604 x2 - almost like opa1611, rather smooth.
So far none of the stock opamp really wow me. Beside MUSES01/02 which I have expected to be pretty good sounding, the only other opamps that really surprised me are the LME49720HA / LM4562 (*they are said to be the same internally), LM4562 in particular is very transparent and clean, giving a similar presentation as O2. It will serve as a great choice for those who are looking for wire-with-gain / colorless style of amplification. MUSES01 and 02 more or less about the same level. 01 has the advantage of having a great balance between detail and texture, while 02 has the advantage of having better layer and soundstage. I'll say 01 is a bit more musical while 02 is a bit more neutral in comparison to each others. Though by themselves, I won't say any one of them is really on the musical or neutral side. They are all fairly well balanced. The opomp I really won't recommend are OPA2209 (or OPA209) and OPA2107. The rest are just unremarkable one way or another.
Thank you! I bet you spent whole day rolling op-amps :)
Beside the source, X3 and DX50, what IEM you used for the test?
I ever tried OPA2209 on my Dr. DAC2 DX, and I was impressed by how clean and detail was the sound signature, it was very good. So my past impression of OPA2209 is contrary with your's on E12DIY. Maybe the matching with the buffer makes the difference. I will test OPA2209 later with E12DIY.