Power adapter 13.5V vs 15V
It is normal for AC transformers to be 10% higher AC with no load than rated load, in this case 5000 mA or 13.5 x 5A = 67.5VA
Due to the loss in efficiency of transformers with pulsed rectified current with poor pf and lots of ripple, this means you'd be lucky to get 10W per channel undistorted, but it is rated for only 8W probably because they went cheap on the x mF caps. ( don't believe the  T6100 MF4086 135W marketing BS for peak in 1ms )
The 13.5V ~ suffix symbol ~, means it is AC output. So you are seeing a normal expected response with no load.
If you are having lots of hum at loud levels, look at replacing the x mF cap with same or better in low ESR.  They aren't that expensive.
If it is static or intermittent on the volume control, try WD40 or replace pot.