high level input

I have a phoenix gold 10” under seat sub. I have spurred a high level signal from one of the rear door speaker’s to feed the signal for the sub. Now I know that some will say take a feed from both sides but to be honest doing just the left was a right headache so can I connect the spur to both...

When I crank my car up my amp comes on but plays at 10% volume for first few minutes then randomly will kick up to normal set volume. Amp's power light is on, all wired connections double checked and good. (Note) I have the sound input running through an external high level output converter...

Sorry if this should be in a different section. I currently have a Sony DSX-310BTX HU feeding an RCA signal to my BitTen's AUX input. I want to put my factory radio back in and use the High-Level input on the BitTen. How much of a downgrade (SQ wise) will the high-level input be vs the RCA?