OP, I would set the subsonic to a minimum of 30 - 35Hz, the OEM speakers cannot handle ultra-low frequencies and you risk blowing them. For the turn-on pop, try setting the turn-on delay on the MS-8 to 5 seconds. That should eliminate most of the pops. However, I still get them when I start the engine sometimes. I have learned to turn MS-8 volume down when I get out of the car, and then up again after it is started.
Mobridge DA2 > JBL MS-8 > ARC XDi 600/4 > JBL Gti 408 Mids & Vifa OT19 Tweeters; PG Ti2 1600.5 > Morel HCW-10 & IDMax 15