Computer audio

I'm running a ASUS Maximus VI Forumla MOBO & have no sound card. I previously had a Logitech Z506 system which served well enough until the speakers finally started to go. After having the mess of a 5.1 system I wanted to go back to a 2.0/2.1 system for simplicity and I wanted good quality, not audiophile, but good, and settled on the Logitech Z623. I know there were better speakers, but I use them for gaming, watching movies, and listening to music (again, not at an audiophile level, but I don't want shite either) and the price difference wasn't worth it to me.

My problem is mainly with music playback, especially tracks with heavy/deep bass and mid-range; I'm finding that the Z623, which should be a huge improvement over the Z506, actually sounds worse in a lot of tracks and no matter what I do (I use Foobar), I can't get it to sound "right."

I know nothing about audio really, especially when it comes to quality PC audio. So, I'm looking for advice as to how I can improve the music quality with these speakers, if that's possible. I mean, should I bite the bullet and switch to another speaker system, should I buy a dedicated sound card, I don't really know about the use of DACs (I mean, I think they're responsible for converting digital to analog - or the other way around - so that you can get quality bit performance; but are they mainly for headphones or can you set them up with desktop speakers as well?

I've tried tons of different DSP configurations within Foobar, I've used Wasabi, ASIO, and the primary sound device, but no matter what I do I can't attain music fidelity that I'm happy with.

Basically, what do you guys think about setting up the best possible 2.0/2.1 PC speaker setup without breaking the bank? Would a sound card make a major difference, is it the speakers, what? I want something with crisp highs, powerful, but clear, mid-range, and good bass (although I AM NOT a bass guy, I listen to a lot of indie rock, punk, etc. and I'd much rather have the mids and highs sounding awesome and have the bass there just to serve out its backup/balancing purpose.

I've read plenty of good reviews about the Logitech Z623s, so do I need another component or could this be an easy switch by replacing them with another speaker system?

Sorry to ramble, I'm stuck at home bored so I may be a little drunk.

But, all I'm really looking for is advice to get quality sound, especially focused on music, from my PC speakers without breaking the bank.

Cheers.