Congratulations on proofing the National Semiconductor®/Texas Instruments® LME49860NA's at the I-V. I'll be checking eBay® for appropriate adapter PCB's to fully tune the same vendor's LME49990MA (which appears to be a direct descendant of the LM318AN high-performance op amp from the equivalent schematic) as a package with surface-mounted compensation filters for the line-level buffer - I have both a Hewlett-Packard® HP2009m display with internal speakers and a Pyle® PCA2 offboard 40Wx2 amplifier for bookshelf speakers that my STX can drive via the dual-RCA output jacks.
Anyone done an A/B test of dual Texas Instruments® THS4031's vs. single Texas Instruments® THS4032 for any one of the three dual-op-amp positions on an Essence™?
You can check out Frisky's impression on THS4032 vs many other op amps including the 49990 on the previous page. He did a pretty nice write up.
I was on the same boat deciding on either THS4032/LME49990 and decided to go for the LME49990. Both are highly recommended but 49990 is slightly more popular. I figured I wouldn't go wrong with either one as an upgrade but my deciding factor was based on the one that offers better workmanship=49990. THS4032 costs a little cheaper but the seller's origin is from China....
49990 is said to offer slightly better low end response than ths4032 but both are known for their superior transparency and neutral sound signature = truely audiophile grade. Also, it depends on what headphone you'll be pairing with. Bass shy headphone will probably want the LME49990 and really bright headphones will probably go well with ths4032 for its smoothness. Well, both shares many similar good qualities.