Wiring speaker signal to amp

[Raph27]

Hello , first post here after search the forum for a while.

The car is 2005 x5 business cd non-dsp.

I'm trying to add 2 subs, i wire the amp following the guide in the main page, everything is good except the speaker siganl for the amp, since the amp has low level inputs I'll need Line ouput converter, my question is: Can I tap the speaker lines that comes from the radio to the factory amplifier, wire it to the rca cable to connect to the new amp? this way i won't need the Loc, is this correct?

Thanks.