You can actually pitch down without slowing down the music, and can even do it live, especially with instruments like electric bass/guitars where you can’t really hear the original pitch.
I believe there are amps that do it, but, if you just want to hear the effect, plug up your amp (at a rather low volume) to the line in on your computer, unmute it in the sound card settings, and apply the pitch shifter that comes with even the built in cards nowadays.
Or just do it with a recording, I guess. But that’s less fun.