Okay, this pertains to running a MP3 player direct through the RCA inputs of a 100 watt car amp (50 x 50) that may date to 70s or 80s, but does have MOSFET technology. I read that you never run an amp past 2/3 volume, but how does that apply to running a battery operated portable MP3 player direct into the amp via the headphone output of the MP3? Do I need to keep the MP3 player limited to 2/3 volume? Can I go a little higher on music recorded at lower volume levels? The amp also has an input level adjustment, which I have turned up to 2/3 of it's max level.