So I've had everything setup for 4-5 days now. Here's what I've found:
The Essence STX is less buggy than the outgoing X-Fi drivers wise (so far), but I can't say the same for the hardware. Sometimes late at night I'll wake up to the relays clicking like crazy, and this morning I noticed that my right speaker no longer had any input going to it, despite nothing being changed. I restarted my computer (figuring it was a driver problem), but it didn't help. When I hooked my MP3 player up to the speakers, the right speaker had no issues. When I switched back to output from the computer, it worked fine. I'll call it a fluke for now; hopefully it won't become a continuous problem.
The speakers are... good, but they definitely need an equalizer for the midrange. Without the equalizer settings in the Xonar drivers, the speakers sounded like they were playing inside a cardboard box - definitely worse than the outgoing Z-5500s, which shocked me. However, when I hooked up my Sansa Clip+, they sounded 100% better. I guess part of the "problem" is the opamps on the sound card, which I've read a few complaints about. Still, with the equalizer set the way it is now, they sound WAY better than my previous setup - extremely clear, good sound stage, highs aren't harsh, and hearing little things in songs that I didn't know existed. The bass can be a bit boomy (only ~4" from a wall, rear ported), but it's far better than keeping the Z-5500s sub set at zero and having no bass at all, lest I annoy other people living here.
Overall, I'm satisfied. I wouldn't recommend the Audioengine 5's to anyone looking for a simple plug and play setup (which sort of sucks, since that's what they seem to be designed for), but if you have access to an equalizer (hardware or software) they sound quite good. They have potential (unlike truly junk speakers which will sound like crap no matter what you do to the signal you're passing to them), but it's too bad the midrange is so muddled out of the box. Would I have been better off with the Rotkit 5 G2's or BX5a's? Probably. Do I regret it? Nah. The build quality is excellent, the setup was extremely easy, and the convenience is wonderful. Full color manual (not that you'll need it), plenty of accessory cables, and supposedly excellent customer support.
Thanks once again for the suggestions guys.