Well, that didn't work....
...... and I don't know why :(
I'm pretty good with ICE as a rule, came up with a cunning plan to fit my aftermarket HU instead of the Concert one (A6 Avant, 2001, non-BOSE). I wanted to keep the sub and amp, but switch the amplification to the fronts to get some staging.
So I got the adaptor for the plugs that includes RCAs -> the amp plug and the remote wire. Then I ran a bunch of speaker wires down the car, and cut into the amp output wires going to the rear speakers. I gave the rears a feed from the HU by joining the new wires to the speaker-side of the cuts, and took the amp output to the fronts from the amp-side of the cuts. The rears came on fine (powered by HU), so I know I got the right wires from the amp.
I then plugged the RCAs into the front pre-outs of my HU and expected to get the sub and fronts come on. No such luck - nothing. The red plug on the sub is getting remote live and permenent live. I've also tried the rear pre-outs, both front and back at the same time, and even the sub pre-outs. Not a sausage.
I've rewired the fronts to the HU for now, so I've got a bog standard, no subwoofer system running at the moment.
Only thing I can think of is if the pre-out voltage on my HU is too high for the OE amp??
Otherwise, I just can't see why this isn't working - even if my new wiring to the front speakers was dodgy, should still have got the sub playing....
......any ideas folks?