Difference in quality between front headphone jack vs rear line-out jack? And would an amp help?

(Newbie here)
When listening to music on your computer, do you plug your headphones into the front headphone jack or the rear line-out jack?  And what exactly is the difference?  I've always had a splitter plugged into the line-out jack on the rear panel of my computer tower, with my speakers plugged into one end of the splitter and the headphones plugged into the other.  Recently I bought a pair of Sennheiser HD280s and initially started out listening to them plugged into the the headphone jack on the front panel of the computer tower...which I found left a lot to be desired.  There was way too much bass and everything seemed very muffled, especially the vocals, like there was a pillow in the way.  I then tried plugging them into the line-out splitter in the back and the muffled sound was mostly eliminated.  Why is this?

From what I've read, it seems that you should plug most headphones into the headphone jack when listening to music rather than the line-out jack, as this apparently is better suited and gives it more volume.  However, in my case the sound is clearly superior (but still not perfect) when listening to it from the line-out jack.  There's a lot less bass but much more clarity and "punch" to the music, and the muffled sound is mostly gone...but neither way provides a completely satisfactory sound (at least not what I was expecting from these headphones) so I'm wondering if I'm maybe I'm doing something wrong, if the soundcard could be at fault, or if the headphones themselves are the issue.  I'm also wondering if maybe a headphone amplifier would solve the problem, either when connected to the headphone jack or the line-out jack.  I've been eying the ART HeadAmp4 amplifier due to its good reviews on Amazon as well as the fact that I could use it instead of the $0.99 splitter I've got now to go from the line out to both my speakers and my headphones and hopefully create a superior sound in both devices in doing so.

So yeah, any help?  Sorry if these questions are really stupid or obvious to most -- I've just recently ditched my crappy 10 dollar headphones and am only now beginning to delve into the world of higher-quality headphones and all the things that come along with them, so I'm not very familiar with a lot of this stuff just yet.

I bought the ART HeadAmp4 amplifier from a local store since they told me if I didn't like it or it didn't work I could return it and get my money back.  I plugged that amp into the line-out jack on the rear panel of the tower and then plugged the headphones into the amp...and they sound exactly the same as when they're plugged into the headphone jack on the front panel of the tower.  I spent a good deal of time comparing the three options: the headphone jack on the front panel, the headamp4 and the line-out jack on the rear panel.  I switched back and forth between the three while listening to the same song on repeat (not an easy task given my setup) and the headphones, when plugged into the amp, sound pretty much exactly the same as when plugged directly into the headphone jack on the front of the tower.  There's a huge difference between using the amp/the headphone jack and using the line-out jack, especially in bass-heavy songs.  The bass still overpowers everything and the vocals and everything else seem muffled when using either the amp or headphone jack and, while it's much clearer through the line-out jack, there's very little bass when plugged directly into it.

Why would an amp plugged into the line-out jack produce the same sound as the headphone jack on the front panel?

I tried my speakers in the amp vs directly into the line out as well but in that case it was hard to tell much of a difference between the two.

Also, to address PC specs: I use a HP Compaq dc7800 Convertible Minitower running Windows XP Pro SP3 and in System Information -> Components -> Sound Device it lists "SoundMAX Integrated Digital HD Audio."  I assume that's onboard sound...like I said, I'm a noob at this.  Also, the speakers are Logitech Z4's in case that matters.

So basically: front headphone jack = too much bass, too muffled.  amp plugged into rear line-out jack = too much bass, too muffled.  rear line-out jack = clear and not muffled, but next to no bass.  Would a DAC like the Xonar DG fix this problem?

Bumping for help

Well I went ahead and ordered the Asus Xonar DG sound card since I was able to get it for $19.00 after rebate, although I'm not terribly confident in my ability to install it properly.  I still need a splitter in order to use both my headphones and my speakers though, since as far as I can tell the Xonar DG only has one jack for that.  Would using the amp I already have (ART HeadAmp4) as a splitter work, considering that it would be basically a headphone amp plugged into another headphone amp?  Or would that cause problems?

Thanks again