Well, "regular 3 pin analog" is not actually regular - it's something you find on PCs and nowhere else. You can purchase AV receivers that come with 5.1 analog inputs, it'll be RCA, and you use adapters. Some of them get very spendy, and there's lots of trade-offs to doing things that way until you spend major bucks (five figures). I do not like analog mch as a solution because of the problems it presents (namely, aside from a few very expensive platforms, there is no ADDA stage for the multi-ch inputs; while this may seem like a "good thing" at first blush, remember that modern AVRs and SSPs do all of their processing in the digital domain exclusively - you lose all of that (which includes bass management, channel leveling, and room correction; that more or less defeats the point of having the AVR/SSP with those removed)). The cheapest solution I'm aware of that does bass management/etc for multi-ch analog is the Parasound P7 (but it does no digital processing or room correction), "up" from that would be the McIntosh MX150, and Accuphase VX700 ($10-$20k), I think the Levinson 502 gets you everything and a bag of chips (and at $40,000, it should).
You won't lose anything going S/PDIF, and you aren't "offloading" processing - you're just changing how the data is sent. The computer is still processing. You need DDL/DTS:C to send a non-Dolby/DTS signal as multi-channel via S/PDIF; I'm aware of NO onboard solution that does this. If you've got a DVD though, the computer should send the AC-3/DTS digital signal straight out and let your external decoder handle it. That said, why bother replacing the Z-5500 at all? What you have works for what you're doing across the board, and you've side-stepped the whole DDL/DTS:C question (and we won't even get into lossy vs lossless, because you're already "there"; same for things like bass management).
Anyways, if you insist, you'll need to find a soundcard that can do DDL/DTS:C - it may be available for your X-Fi with an upgrade pack (few dollars driver). Digital out is via TRS; you just take a TRS to RCA adapter - I can never remember if white or red is out (one is out and one is in, try it one way, if nothing, try the other way - you won't hurt anything plugging an input to an input, you just won't get a signal). If you need TOS, get a coax to TOS adapter (few bucks). Then you'll put that into your AVR, and let it decode. This won't "hurt" for games (they use lossy audio anyways), for music you'll want to run in stereo (PCM, lossless) and make the AVR do the matrix up-mix (if that's your thing; basically if you want to take stereo into 5.1, do it in the AVR, so you're sending PCM @ 1411 vs DD @ 640; Neo:6 is my pick from the commercial stuff). For movies it should do passthrough, I know VLC can be configured this way, I believe WMP can be as well. No idea about other players, but basically you want a "S/PDIF Passthrough" mode where it bitstreams the AC-3/DTS audio out to the receiver.
Again, I'd just keep the Z-5500, as it sounds like everything works for you, and any HTIB package you buy will have similar quality speakers. If you're stepping up to better speakers with the upgrade (and a real AVR), that's another story, but that's also a lot more bucks.
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EAX is not needed. I've disabled it anyway. I just need to be able to output 5.1 surround to a HTS. And I've searched a few forums with people complaining that S/PDIF on this motherboard only outputs stereo to their receivers :/ Why dont they just make HTS with regular 3pin analog connectors?
