Here's my 2 cents from what i remember from my audio install on my volvo...
My Volvo c30 had the premium sound system by dynaudio with 10 surround sound speaker. i wanted to just add a sub for more bass but some dealers said no u cant do it we must change ur splits also bla bla... But i went to a good installer and they explained what they could do so that i can use my normal speakers powered by the stock amp and still add a sub was to:
1) add a splitter to the current harness to they could tap into the sound i think its like the jl clean sweep. i cant remember the name but it wasn't expensive. Then what we did was put bass blockers on the stock speakers so they had no bass. They then connected the sub and to a separate small Rockford amp i had and voila they sorted it.
I had full use of stock HU and the sound was amazing and i pumped it all the time and not 1 problem to the stock speakers as they were still only running on the stock amp.
im planning on getting them to do an install on my 370z as well but just add one 10" or two 8" Rockford Subs.
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09 7AT Touring 370z, Pearl White, Evo-R Carbon Fang Cover, Carbon Top Cover, HKS Centre Pipe, Illuminated Kickplates, Custom Sprayed stock RAYS 
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