no sound

First of all, what did your friend do to the system?  Some friend.  :)

If the amp is receiving power and turning on but there is no output from the speakers, you have either fried the speaker voice coils or fried the output stage of the amplifier.  It is VERY unlikely that you have "fried the rcs inputs" unless the cables are physically damaged somehow.  Disconnect your subs and try a different speaker temporarily connected to your amp just to see if it is the amp or the woofers.

maube you should take a test light and check the output on the amp while the amp is on. If the amps turning on you know your getting input, see if you get any output coming out of the amp.

I have never heard of frying your inputs. I would take any advice I could get from DYohn.

he doesnt know what he's talking about........ the input stage of an amplifier is essentially bullet proof, you are dealing with audio signals going to low voltage op amps and pre buffers... you can dead short an RCA input to ground and the unit will simply go dead one channel ( dont try this at home ) thats all a balance control does is take audio signal to ground......

you may have pooched the power supply in the amp or possibly the OUTPUT stage ( although you generally know when you've done this... smoke, fuse blowing , etc. )

BTW, unless you live in Guatamala, you aren't gonna get an amp serviced for $30.00......    here's an idea.. give your " friend " 30 bucks and tell him not to come home till the amp is fixed and working......

I agree with the above comments.  If someone says they can "fix" your amp for $30, check to see if there's a blown fuse because that's about all $30 would fix.  :)  If you are sure your head is sending a signal to the amp, and you are sure the RCA cables are not damaged, and you have tried another speaker on the amp outputs and got no sound, and the amp is ON (fuses are not blown) then the amp is probably fried.  The OUTPUT stage, not the input.

I suggest you go to a good car stereo installation shop (not a Circuit City, but a shop that only does car stereo) and ask them to test the amp.  Now THAT might cost you $30, but at least you'll know what's wrong.

Audiobahn equipment is not one of my favorites precisely because I have seen lots of it fry for no apparent reason.  Some others on here have used it for ever and say it works great.  Whatever, every piece of gear is different.  But your story does not surprise me after looking at the brand name you are using.  Sorry.

Depending on the model head unit, if there is a sub control on the head unit, make sure that is A) on or B) turned up that has been a stupid problem i have seen. Its really quick to fix, and makes you look dumb. He may not have know what he was doing when playing with the HU and accidently turned the subs down, also find out when they stoped working, if they just decided to stop working while he was listening, then its probably the amp(99% sure it would be), if he was messing with the HU, odds are he changed a setting to make them stop working. Check with him, narrow your possablities down to one source. If you have to even just try a different amp like you did the subs. That will tell you quick.

I would look at the setting on the HU first

Mark