I have the same set-up as all of you. I added a sub with absolutely no problems. The Signal to Noise ratio of the stock head unit seems fairly well. The unit I got is made specifically to interface with the OEM Bose system, made by PAC Audio. Their web-site is www.pac-audio.com
. The part numebr you need is the AOEM-NIS2. This kit comes with everything you need to tap into the audio signal between the head unit and the external Bose amp. This output converter will give you 4-channel full range RCA pre-outs. Though I am only using this for a sub out, I had this same unit previously installed in my 2003 Nissan Murano w/Bose. With that set-up I had replaced all the speakers and added subs and the sound quality was great.
Anyhow, I'll attach a diagram of what the set-up looks like. If you have Navi...use the rear RCA's so when the Navigation starts talking it wont fade out the subs like it does for the front speakers.