what i originally thought of recommending-- i use a couple of them in my own (carputer ) install, and they work great.
but, with a ipad/pod/touch install, you have a couple different options.
this hifonics unit looks pretty good for a ipod type install, and includes a line driver, and a little bit of eq to fine tune the sound..
or, if your amps have a healthy signal already, and you are happy with the overall sound(don't feel the need to adjust the sound), this is a volume control that ties into the rca cable.
the advantage to that is that it is cheap, the disadvantage is that you need to route the audio cables wherever you want this knob mounted..
there are other similar solutions out there by other companies, these are just a couple of anti-head unit ideas i got
now, quit paying attention to us, and enjoy your vacation!!!