by jack to usb converter are you referring to an external usb soundcard?
number 4 on this photo http://g-ecx.images-amazon.com/images/G/01/electronics/detail-page/B002I3OZB2_G330_Illustration.jpg
does the logitech have any software? if so it may have an equalizer. i would suggest lowering the left side of the equalizer which should cut out the bass.
if it doesnt, you can always get an equalizer program for windows and use that and do the same thing. this came up in a quick search http://sourceforge.net/projects/equalizerapo/
but i'm not sure if that is the best one for you or not.