Multiple Radio Pro Interfaces Dead/Dying (!)

2006 Prius (Gen 2) with JBL - 6 CD changer and NAV. I had a professional installer replace my head unit with a Sony XAV-AX5000. They used a RadioPro RP4.2-TY11 interface unit (like a lot of people on this forum).

After the initial installation, everything seemed to work fine at first (including SWCs). However, eventually (after about a week) there was no sound. My daughter said the sound was "crackly" before it died. When I took it back, they fixed it by replacing the interface.

Now the sound is going bad again - it's getting static-y and the volume is getting much quieter (seems like almost no bass, too). I have to turn the volume up to get normal volume, and sound sounds thin/weak. I have a feeling this interface is going to go bad, too.

Does anyone have any suggestions? Is this just bad luck, with two interfaces going bad? Or could there be something in the installation that is causing this?

Thanks for any help.