You could get the Fender Fontman + M-Audio bass traveler. This nifty little amp also includes bass switches; or you can shut them off and listen in a fairly neutral manner. It’s not hi-fi, but it works.
Products form this company might also work:
For your intended application, of course, not ncessarily hifi / summit-fi, but then again, who wants to hear all the bad parts of music if it’s too accurate?!
Which headphones are you using? Maybe I could recommend you something better.