I wanted to do a theater install in my car I know at one stage alpine made an amp with digital input.
Is there any other amps that have digital inputs or are 5.1?

 

there must be because i see a lot of headunits with digital out

 

I'm not personaly aware of many amps with digital inputs. I think the digital outputs on the headunits are more for people who have processors, like the Alpine PXA-H701. I believe Pioneer also has a unit that has an optical input, but I believe it needs to be used with the proper Pioneer headunit to work.

From the processor, you can then run very short runs of RCA cables to your amplifiers for each channel.