Kicker zx700.5 sub channel has humming sound and amp2 has echo like sound

[ivkata]

I've been wondering back and forth about this amp as I'm unable to find solution for the problems I'm asking here are in the past i always find help and good advice
here is the deal i have kicker zx700.5 2007 says the board its red one .amp powers on and had good rail voltage it appears to have never been worked on .it came to me for free as i sold good amp and installed in the buyers car so they have me this one as they have no use for it .
problems i see .Amp2 outputs audio that sounds like voice is faded almost like far away echo .compared to the other good working channels i can see lower output as well
second issue.sub channel dosent output anything but humming sound.if the Rca cables are not connected there is no humming sound .regardless of the type of signal i drive into the amp the output is only humming

i have reflowed the connectors between the main and driver/input boards, rca have been reflowed as well.On the driver board/input board i see both positive and negative 12 volts so I'm thing a bad op amp maybe

please advice

[ivkata]

the mystery gets deeper .I was testing the amp with my iPhone like all other units i sell. I did connected a Alpine cda-117 i got for my car and used all 3 setts of rca to feed all the channels and now the humming noise is gone if i leave one Rca out noise is back .I never had this issue with any other amp i sold .this is a good amp i wanna sell it a reasonable price without someone being unhappy with purchase as i do sell from my home

[Perry Babin]

Not all amplifiers will work with MP3 players with floating grounds.

[ivkata]

Learn something new every day .thank you for the response Perry