Need help with a line level convertor install.

[BlackTerri10]

Hello
I am only posting this in this section because i need an answer because im installing tomorrow,
i have a Territory with premium sound, i want to Add a new monoblock amp for a sub woofer, i also want to keep the factory amp so when i have to use the rear seats i can take my sub out and then use the factory one.

My Question is

i bought a line level convertor from jaycar http://www.jaycar.com.au/productView.asp?ID=AA0482
and he told me that if i tap into say the front speakers then i wont be able to use my front speakers.
I thought you just tapped into a speaker line and the speakers will still work,
when i asked about this he said you cant do it.

So is he right?? or can i splice my line level convertor in and be all set??

thanks in advance.