Before you do send the amp back try these tricks:
1: Do you have any video components tied into the system if you have a cable box they can have different grounds and can require a ground breaker
2: Did you check your dimmers for lighting these can sometimes cause a ground hum.
3: How did you test floating the ground? At an power conditioner do you use one?
4: Have you methodically tried floating the grounds one at a time, on different components?
5: Did you test without any components connected to the amp?
6: Have you re-introduced cables one at a time? Sometimes you can have a bad ground on an RCA cable
1: Do you have any video components tied into the system if you have a cable box they can have different grounds and can require a ground breaker
2: Did you check your dimmers for lighting these can sometimes cause a ground hum.
3: How did you test floating the ground? At an power conditioner do you use one?
4: Have you methodically tried floating the grounds one at a time, on different components?
5: Did you test without any components connected to the amp?
6: Have you re-introduced cables one at a time? Sometimes you can have a bad ground on an RCA cable