I have some personal experiences of all those four op-amps to share. I'm impressed they all sound so different. Mostly elegent sound is Burson Audio V6 Classic. LME49990 is a bit more analytical, focused to more micro details and clean sound. BA V6 is more personal sounding and softer. It makes your inside happier. OPA2604 was kind of nice but there was some problems at details and transparency. It was a bit weird there. For my take LME49990 dual to mono is super nice for the price. It's better then orginal op-amp and does not cost to you arm&leg. But if you want pure joy and don't care how much it cost go for Burson Audio. There is available Classic and Vivid. I only tested classic version. I test in near future too Sparkos op-amp. So my impression my change or not
LME49720 is around a dollar piece. LME49990 dual to mono is 8 dollar. OPA2604 is 50 snt. Burson Audio op-amp is 80 dollar a piece.
This impression was with Yarland FV-34C amplifier and Quad 12L speakers.
Dynamics
BA V6 Classic
LME49990
OPA2604
LME49720
Color and texture
BA V6 Classic
OPA2604
LME49990
LME49720
Transparency
BA V6 Classic
LME49990
LME49720
OPA2604
Details
LME49990
BA V6 Classic
LME49720
OPA2604