By the way the LME49990 spec sheet calls for at least double power supply bypasses. The STX does not have the recommended bypasses stock so may not sound as intended if mounted to the STX & as such may actually have some emphasis in the bass as a result of less well delineated high frequencies compared to if the proper bypassed were present on the STX card
That may be so in theory, but it's not how it works out in reality, LME49990 sounds audibly cleaner in all frequencies ie higher resolution of detail, easier to detect tiny tiny details, room placement of instruments, reverberation, no irritating s-sound sibilance etc etc stock sounds really dry, flat and boring in comparison. It is possible the LME49990 would sound even better with proper power supply arrangements but it is superior to stock even when the implementation is half-broken :-)
I would be very happy to be pointed in the right direction on how to enhance the power supply on the ST/STX, a step-by-step guide would be amazing, is it doable without having an oscilloscope etc?