Adjusting the sound of bass in Windows 7

I have a wireless radio transmitter plugged into the line-out port of my computer.  The transmitter broadcasts to my wireless speakers which are scattered throughout the house.  They are quite old but work well.  I like to listen to internet radio (Pandora, etc.) with them.  However, on some tunes, the bass is much too loud and becomes distorted.  There is no bass adjustment on these speakers, nor on the transmitter.  I can adjust my computer speakers (not wireless) by physically turning knobs, but of course those don't influence the wireless speakers.  On my old XP, I was able to bring up bass and treble adjustments by right-clicking the volume icon in my tray where the volume is adjusted.  However, I can't seem to find any bass or treble adjustment there.  How can I turn down the bass on the line-out port so my wireless speakers will have less bass volume going to them?

Thanks for your help!