I guess my new project over the next few months will be how to get the most accurate sound for music from this dedicated PC for under $600. It sounds like I might do best to go analog out from the X-Fi to a descent stereo reciever (remember, I'm on a budget so a pre-amp/amp is likely not affordable) ...
There are couple of good names (aka, companies that don't sell underpowered amp units) in the receiver section. H&K and Pioneer tend to come out top in the analog section.
The most popular receiver lately has been the Panasonic SA-XR57/55. It's a "digital" amp (Class-D, uses high frequency ("inaudible") switching which allows signal processing without DAC-ADC processes). For stereo it's quite a gem, and uses dual-amps (per channel). It measured at 93W/channel in stereo mode at 8ohms, and maintained 93% of the current down to 4ohms and 88% at 2ohms. The Class-D nature of the amp means very low noise levels (0.03% THD with digital input, and 0.04% THD with analog input at clipping; these are similar numbers put up by the Denon 3806, which is closer to $1000, and isn't dual-amp). The 55's retailed at $230, and the 57s should come down to that price eventually, although it seems early low supply and high demand has them closer to $350.
to descent 2.1 system (?). I'll do some research and see what can be had for $500...maybe even $600.
Some suggestions.
(1) If this is going to be in a bookshelf (enclosed rear end), I would suggest the acoustic suspension (non-ported) SVS SBS-01s for $220/pair. You can check them at out www.svsound.com. The same SVS that sells world class subwoofers (along with Hsu Research). The SBS-01s are plain black have a 1" soft dome and a 5.25" poly cone, along with the sealed enclosure means bass may be a bit lacking (although...not compared to PC speakers).
(2) A 2.1 set for $400? www.av123.com sells the X-series X-LS ($200) and the X-sub ($200). The X-LS is a ported bookshelf with real wood veneer, a 1" soft dome and a 6.5" midrange cone based on the $800 GR A/V-1 bookshelf kit. Thanks to the higher volume of AV123 products (compared to Do it Yourself), the AV123 only pays $110 for the X-LS drivers. That's the beauty of internet direct speakers. 55% of the cost alone is in the drivers. The bookshelf is listed at 60-27KHz +/-1.8db (according to Audioholics measurements). The X-sub is flat to 28hz (+/- 3db).
(3) My personal speakers, the Ascend Acoustics CBM-170SEs. (www.ascendspeakers.com). They are a bit more expensive, and it would be far harder to fit 2.1+amp into a $600 budget, so it's just a higher priced option I'm putting out as a comparative example. They are $350/pair, and are built with SEAs 1" soft dome tweeters and 6.5" polygel (kevlar+carbonfiber+poly) composite drivers. They measure 60-20KHz +/- 1.1db. Obviously, if your starting point are the $200 SBSes or X-LSs, this is not the most massive upgrade, but 1.4db tighter accuracy is nothing to sniff at, as human hearing is sensitive down to 0.1db increments.
The aforementioned $220 Logitech Z-5500s are measured at 200-20KHz (I assume the bass was way too inaccurate to fit into the measuring window) +/-4.95db, merely for reference, via Audioholics.
None of these are retail brands. Retail brands of course, can't touch internet direct speakers for accuracy, but are more available (on the floor in stores). All of these speakers have 30 day return policies (although you have to pay one-way shipping--to your home. It's ~$16 to test these speakers basically).
Just a few suggestions. If you would prefer retail, I would say Energy is the most neutral brand (the C-3s can be had from $250-$500 retail).