pre out impedance match avic to oe jbl

We put a AVIC-F700BT in a 2009 Kia Rondo with the factory 7 channel system (4 + center + dual VC sub). The OE amp is branded JBL and KIA. There's a very annoying hiss at all volumes and poor low frequency response. I'm sure the hiss is a ground loop issue, but before buying a set of isolators I'm curious about the following.

From the kiatechinfo site, the factory HU (M466) put put 3.2v 10k ohm impedance to the factory amp. The AVIC-F700BT pre-outs (which we are using to drive the OE amp) run 2.2v with 1k ohm impedance. Does this mean the JBL will never get to full crank with the AVIC?

With absolutely no hands on experience with this sort of thing, a perfectly reasonable thing to do seems to be to find a 3:2 (1.5:1) audio transformer with 10k secondary impedance so the amp sees 3.3v at 10k ohm. This would also get rid of the ground loop.

Am I over-complicating it? Thanks, JSD

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No engine/ignition noise or alternator whine, it's even when the engine is off.

By ear (using test CD), it sounds like bass drops off around 150-100 hz. Don't know if that's a result of the mismatch or a limitation of the OE amp.

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A Pac-audio.com, SNi-1 is a ground loop isolator.  It is a transformer with a 1 to 1.3 (I think) ratio.  They sell for about 20 dollars.  You should be able to find a local shop that will let you try one to see if it will help your problem.  Most premium sound systems from the manufacturer have equalization built into either the amp or the radio. The equalization is designed specifically for that vehicle.  There is a good chance that the equalization was built into the deck. 

Perfect - thanks!