just curious if its going to give the same quality
no way
however since your just using it for subs, you probably wont be able to hear the distortion, but if you dont want to replace the head unit, then you got no choice. there all pretty much the same, some just cost more
well there is another option, but its definitely in the advanced class. you could solder RCA's onto the factory amps signal in wires (even if you dont have the factory subs the harness should still be there) then you would have a rca cable that is getting a preamplified signal from the headunit, which is about as good as you can get from the factory radio. only thing missing is the level control, which this will take car of (if your amp doesnt already have it)
http://www.ritzcamera.com/product/EP79482663.htm?utm_medium=productsearch&utm_source=google
you will need to boost the remote turn on signal if you dont want to run a wire from an acc source (most ACC sources dont stay on with the RAP: Retained Accessory Power)
http://www.bestbuy.com/site/PAC+-+L...ci_sku=9046098&ref=06&loc=01&id=1218012117328
then you can do the entire install from inside the trunk and never have to go inside the car for anything
metra also make a line output converter that uses signal sense to creat a amp turn on signal, but it cost more than a regular LOC and a TR4
such as this
http://www.big5electronics.com/catalog/product_info.php?manufacturers_id=29&products_id=2587
this is pretty much the same thing that NYC linked to except it has the signal sensing circuit built in
but if you really want sound quality, this is the best line output converter on the market
http://www.rockfordfosgate.com/products/product_details.asp?item_id=106187
but then again for that price i would just replace the headunit.
also if you do tap into the factory sub wires (post amp) for the LOC, make sure that you get one that can handle all the extra power the factory amp kicks out, its a lot more than the factory radio's put out that most these things are designed to be used with.