i'm just not sure of those headphone specs. it says that they work with phones so they shouldnt be hard to drive or cause issues but i cant be sure without knowing the specs.
it could very well be that the headphones are fine and that its just a very poor soundcard which is having troubles on its own with your eq settings (which look a bit too high on the low end to begin with)
no, you wont be able to replace the amp on your soundcard however it could be boosted by using a headphone amplifier. unless i can verify that your headphones need one i will assume that they do NOT and that its just your soundcard being junk. you can read why i wont recommend a headphone amp unless your headphones fit the need for one in the following lines...
do read: if your headphones arent hard to drive and are not the issue then a headphone amp can blow your headphones. they are not meant to be used with normal headphones. amplifiers are used for the more powerful sorts of headphones such as "studio headphones" which often arent powered by normal soundcards.
you can buy external soundcards. they connect up via usb and either stick out of your port (the small headphone 2.0 only types) or sit on your desk (the larger better quality ones)
http://www.newegg.com/Product/ProductList.aspx?Description=external%20sound%20card&Submit=ENE
i can vouche that most products from creative or asus are pretty decent (well above the absolute cheapest models) however i'm not sure if you want to spend that much. if you go with something from a no-name brand or low end product you could end up in the same boat you are now.
if you cant find specs on your headphones... try playing them from your cell phone or portable device. if they play fine and can hit high volumes and even handle bass well then they dont need to be amped and its probably just a case of a poor soundcard or you going too heavy on the eq settings.