if you're using windows, it has a built in mixer than you can control volumes of all the programs you're using.
overall, i dont get what you're looking for. are you looking for some kind of an external device that will control your volume from your computer to your tv? or are you looking for some kind of a EQ software that will even out your overall sound? you can try to raise the bass on your system but you're just raising the bass. its pretty obvious to why everything else sounds more quiet when only thing youre doing is raising the bass, because only thing your doing is youre raising the volume of the lower tones and those tones just sound louder, or everything else just sounds more quieter. if youre problem is frequency, you have to do a better job at EQing your system and not just play with the bass.
if thats not the problem... maybe you have your speakers plugged into the wrong jacks. so your speakers arent reproducing what its suppose to be reproducing, but instead, its reproducing what side monitors, or rear monitors are suppose to sound?
the question just seemed really odd to me.