The port mounting might not make as large of a difference as you think. If the ports are very close to the rear surface, they could get acoustic loading from that surface, but it's hard to say how much. A lot of the bass boost you get from placing a speaker near a wall is simple boundary gain, and it doesn't matter where the port is facing for boundary loading.
Here is a guide to improving the sound of a desktop audio system.
If the problem is too much bass, just EQ the speakers! Since your computer is a source, download something like this
Equalizer APO and if you want to make it super-user-friendly, run the
Peace extension on top of it. Buying a new speaker is a terribly inefficient way to equalize them, which is what you are really trying to do here.