Factory Bose Head Unit to JL Amp connection
 

Hope in right section here. So I chose to keep factory head unit but now having second thoughts. I have to turn the volume up too high to get the level of bass I like. Wondering if it's due to not enough voltage in the signal going to my amp?

Bose head unit into PAC SOEM-4 line out converter into JL amp. I've also got a bass knob to adjust the subs. I've learned that aftermarket head units have 4 volts on their pre outs. Maybe I'm not getting 4 Volts to my amp?

Source is iPhone 5 cable connected through USB. An idea was to try cd but haven't gone there yet.
Any and all help is appreciated. Thanks!

You nailed it, a line out converter will not produce signal voltage to the amp.
2 ways around that, you get a new aftermarket headunit or get a Digital Sound Processor like a JBL MS-8 that is capable of converting the signal to 3 volts.

Thanks. That MS-8 is a fair bit of money. I figure for a bit more it would be better to get a new head unit and start off with a strong, clean signal. I'm going to check the voltage at my RCA's next chance I get. I'm just curious.