Help In Adding An Aftermarket Amplifier, 04 M45

[kentb]

what is the cheapest way to add an aftermarket amplifier to power a Sub for a 2004 Infiniti M45?

I was looking at getting a Lineoutput Converter and just splicing the wires on the factory amp..

But if i connect the Line out converter after the factory amp, it might have a high pass filter. and if i tap into the factory subs, it might not be too loud(the more you turn it up the sub volume decreases)

But i asked for a second opinion on a local shop and they said the only way to do it is to get a $120 module.

what do you guys think?

Thanks!