Upgraded Audio - AMP and Speakers - Now Tweeter Hiss

Ok, so I have a 2012 Prius III. I had a pro shop install an alpine MRP-F300 amp and new alpine speakers. They used the speaker output direct to amp, then amp to speakers (standard setup). There's quite a hiss in the tweeters with the gain set correctly. Good pro shop, I brought it back to trouble-shoot. They installed a few different RCA line converters, tried a few things, and weren't able to make the hiss go away. Stock head unit. Has anyone run into this? What did you do to fix it? I trust that the shop has everything properly grounded, etc...