If you are planning on sticking with 2.1 (as you choices suggest), for roughly the price of the z623 and Asus card together you could get the Cambridge Audio Minx M5 ($229) which is 2.1 over USB.
http/www.cambridgeaudio.com/products/minx-m5-usa
This should be an excellent sounding system that should be quite a bit better than the Logitech. It's basically a slimmed down version of their Minx line which is an excellent sounding line. On top of that, since it's connected via USB, you don't have the normal distortion that comes with line jacks. With this line you will be getting a sound that has greater clarity and dynamics while providing a far better soundstage that what the z623 can give you.
That said, according to Cambridge it only goes down to 45hz, which is usually at +/- 6db for items where the +/- isn't given so you are looking more at about 50hz +/- 3db. Though the Logitech are listed at 35hz, again the +/- isn't given and about the only review I've seen that tested the range was here at Tom's and the roll off looks to be around the low to mid 40's mark.
So the z623 will probably go a little bit lower, given your preference it could be a bit of an issue. However, the M5 will just sound better for everything else.
Now, the caveat is that this is brand new, literally. It just started selling online within the past 24hrs. Given that this is based off their regular line (just a bit smaller), we can infer the sound these should give you but as such, there's no reviews out there.
If you are interested in these, here's a link to the minx thread over at AVS so you can see what this line is about. Hopefully someone on that thread will get a set somewhat soon and give their impressions.
http/www.avsforum.com/t/1339511/cambridge-audio-minx-loudspeaker-discussion-thread
Cambridge does offer a 30 day audition of there products through the online store if you think this is the way you want to go, just be aware of the terms and conditions for it. And if you do, make sure you break them in before deciding on keeping them or not. It'll take at least a good 30 hours before they settle in and there will be a noticeable difference, just don't over work them during that time.