I don't know if this is out there. It would be nice for guitar and bass players that are in areas where we can't crank up that Marshall to 10. Is there an app on your phone that you plug into your guitar or bass into the docking port of your phone. Listen to what you playing through your headphones. You have your choice of amps and pedals? If not make an app and I'll take 1%
There are tone applications that I'm sure you can hook headphones up to.

Aside from that, a lot of amps or effects processors have some sort of extra output port. My RP-155 has a headphone output, and the amp I use for practicing has a quarter inch output. You should be able to find a headphone to quarter inch adapter piece, or just buy some headphones with a quarter inch piece.