It sounds like you just spliced the two harnesses together and didnt use the deck's preouts. The 9-5 has a factory amp so you need to splice RCA connectors onto the wiring harnesses speaker out puts and plug them into the deck's preouts. There is alink to a site with all you need to know about this. Im on my phone ore id get you the link, i think it might be in the sticky above, otherwise Im sure someone else will post the link.
I'm You don't want it to be loud? And what's loud the sub's (aftermarket or stock) or the other speakers? The stock speakers aren't 4 or 8 ohms so with an aftermarket radio they sound like crap (low) it only sounds good if you don't like your music very loud. So if you installed aftermarket sub's there're probably 4 ohm speakers and will sound very loud with the new head unit. For this reason I stuck with the stock head and just added an amp and 10" subs. I plan on changing my head to an aftermarket but only when I’m ready to change all my (stock) speakers, right now how I have it set up it sounds fantastic; so I’m in no hurry to change it.