The real problem with the Z-680's, and yes I auditioned them in my home before I made the purchase (good friend had them, paid him $20 to bundle them up and bring them over for a couple of days)

I've tested the following (previously I had Klipsch ProMedia 4.1):

Klipsch Ultra 5.1
Logitech Z-680
Logitech Z-5500 Digital (bought these before the Klipsch and returned them)

Anyways.. The Z-680s bass is too pronounced, I believe the crossover is set at something like 65Hz resulting in more bass coming through. I personally have my Dayton set at 45Hz and the Ultras come stock at 45Hz.

I mean, if you like car audio bass boom shit, then the Z-680 or Z-5500 is for you, but was just too much bass. (Yes, I have two subwoofers, but that's to spread the bass around my room)

Plus, I'd rather say that I'm rocking 3 subwoofers (dual 8" in the Klipsch, 1 10" in the Dayton) than one

Anyways. The Klipsch have much better midrange and highs, they make MP3s sound like shit, in a good way. It made me convert to lossless OGG and I've never looked back.

I admit, I am somewhat of an audiophile, but only in a "budget" way. I found it more cost effective to buy the Ultra's (then upgrade, and sell the stock satellites) and the Dayton than to buy a real home theater system with a receiver. The DDTS-100 doesn't do a whole lot, but what every HT needs (decoding of all possible digital formats, upmixing stereo/analog to NEO6, 7.1 compatability, etc it does exceedenly well.)