The low level hook up is better if you have that option. I was not sure about your sony. If you hooked the amp up the high level way the the HU speaker wires dub as remote power (in)and the remote power (in) on the the amp turns into a remote( out) for adding additional equipment.
Sinse you use low level input , this isn't your problem.
I got my amp's gain set between 4 & 5 and can turn the volume all the way up , adjusting base only . I'm still running the HK HU and the older Hogtune 5x7 speakers.
2010 FLTRX , Stage 1, 13" Madstad, Mono tubes, SuperShox, Heritage style bars. Alpine CDE-HD149BT,Soundstream Picasso Nano Series PN4.520 , 5X7 and 6.5 Infinity Kappa speakers
If I have to explain ,you wouldn't under stand anyway.