I don't blame you, I'd send it back and look for a better unit from another seller.
I've seen a good one myself, If and when I choose to upgrade my current unit there is a choice of two I'd go for. I'll have a look later when home and copy link to the two I'd pick. These have all the bit's that I'd want on top of the bits yours already has.
Think we've hit a wall with these units, looks like Marky may be in for a world of hurt!
Saying that what you can try is a little trick, stick an extra earth to the unit!! You've got loads of pre-drilled holes already, just stick a wire on and earth it somewhere behind unit, see if that fixes the issue! If not, you've not altered the unit and send it back.
The noise you can hear may just be from poor earth when volume is up high?!
Give it a try!!
