Audio is a chain and it is only ever as good as it's weakest link
For digital audio the chain is as follows
Speakers (Quite a variety of things go in to their quality, let your ears be the judge)
cables (guage and distance matters, everything else is marketing)
Amplifiers (A spec sheet for an amp tells alot about it, but not everything)
Interconnects (these are a bit more important than speaker wire, but this wiring is often internal w/ an AVR)
DAC (Probably one of the more overlooked components by the general population)
Digital Interconnect (Digital cables work or they don't, don't spen a dime more than you have too, construction quality does count for something though)
Digitial Source (16 bits gives about 100db of dynamic range and 48khz sampling rate means that it can reproduce 1-24khz, so 16b/48khz is pretty much good enough)
So if you drove you Z5500 in analog mode from a Xonar you'd get >16b/48khz audio (not that big a deal really), Great DACs and then you'd pass that audio to your Z5500 amplifier and speakers, leaving the weakest point you speakers and amp.
You could power the Z5500 with a real AVR (Although the xonar has DAC's in the same range as 1500+ AVRs) and then upgrade to real speakers later.
My setup to replicate with new parts for just stereo (2 fronts, sub and AVR) would probably be $2000-$2500 and I've compared 24b/98khz to 16b/48khz of the same material back to back and basically could not tell the difference, so the kind of gear you must need for that is way outta my price range and probably yours.
I probably wouldn't even worry about anything until you had some coin to drop on an AVR, then you can replace your Z5500 speakers peice by peice, then you can go back and start upgrading all over again (which is what I did).