Yeah, I'm aware that I don't need to spend this kind of money for casual PC gaming, but I've got money to blow and I haven't bought enough shiny new toys lately
Well, I've gamed non-competitively on my 880 for about a year now and I haven't found the "blind spot" to be a bother--in fact I don't notice it, which is probably why I'll never be able to go pro, haha! There's enough bass in the 880, and if anything, you could always run it through a cheap DJ mixer (like I do, with my Xbox) if you want to fiddle with EQ knobs. I did for a bit. Now I just can't be bothered and leave it flat.
In the end, though, if you're just playing for fun--whether online or offline--I don't think you need to splurge a whole lot on amazing headphones. Was just at a LAN shop yesterday playing MW2, and their cheap Logitech headset (looked like this one) was just fine for locating people running nearby. If you do spend, the reviews here and elsewhere on the 'Net would suggest a good decoder is more important, such as the Ear Force DSS or Astro MixAmp.
By "advance audio processing" do you mean Dolby Headphone? I thought the RCA outs were analog, and you'd only bypass the card if you used the digital out. Apparently I don't know as much as I thought I did.
If you hook up an external amplifier (HiFiMan EF5) to the RCA outputs of the Essence STX, you will bypass advance audio processing of the Essence STX and you will only get a really nice sounding stereo 2.0 sound.
Better to upgrade the 3 op-amps of the Essence STX and use it's built in amplifier.