PurpleAngel, LM49990 are on the way right now, I've ordered them soldered on dip 8 adapters (two chips for each adapter)
I can tell a little story here about STX harshness: this problem is really apparent through my main system (DIY amp based on TA3020) than my headphone (sennheiser HD540)
That's because the TA3020 is very very revealing, given a proper source it will deliver a fantastic sound, dynamic, fast, super detailed, strong bass, but it show any flaw in the source, and can't tolerate HF noise
This is common to all high speed wide band amplifiers, they amplify also HF noise, running hot and deteriorating performance in audio band
I discovered this is the same reason for blamed poor bass performance of LM4562; try to clean out the signal from HF noise, and it will deliver a spectacular strong, heavy and punchy bass, without that sense of coldness in the treble
I had major benefits working around output stage feedback loop (that is an active filter in the end) and filtering the PS, particularly the 3.3V supplied by PC mother board that fed also the oscillator
Next to the PCI-ex slot pin there is a little ferrite bead to filter the 3.3V; I replaced it with an hand made coil with toroidal ferrite core, and I added a 47uF X5R ceramics cap in parallel to the line
This mods removed A LOT of the harshness and provided better instrument definition and sound stage, maybe a lower noise power reduce oscillator jitter ?