New HU and no sound from rear speakers (but feeble output from sub)

Hi
Hoping there's someone out there who can help on this. I've just fitted a Pioneer DEH-P65BT in my B6 A4 I think not bose (see previous post http://www.audi-sport.net/vb/showthread.php?t=52539 ).
However, whilst the front speakers work ok, there is nothing coming from the rears and only the tiniest bit of bass coming from the sub. I know the standard sub is meant to be a bit of a pussy but this is SO quiet you really have to concentrate to hear anything.
I know from reading other threads and advice from AndyMac that the amp needs to be powered so I've connected the power lead to the power antenna lead and am using the autoleads adapter PC9-404 which connects the amp plug to the HU via the pre-out sockets.
I've no idea why this is happening - I thought the rears and sub should work (albeit less powerful than the fronts). Don't know whether it's relevant but the new HU apparently can run a sub directly (it's called direct sub drive apparently) so could it be that the amp isn't switching on but the feeble output from the sub is due to the HU? I'm planning on wiring the rears directly eventually but for now would like to hear something from them and at least as much bass from the sub as I had with the original HU. By the way, I have played with the sub controls on the HU and put all levels up to max but hardly makes any difference. Any help anyone can offer appreciated.

OK I'm guessing I'm not the only one stumped by this. Something else which may be relevant is the radio doesn't pick up many stations, so one theory I have is that since the rear amp and antenna amp are connected to the power antenna lead coming from the HU then could it be that this is where the fault lies ie. both amps are not getting power? (I think I read somewhere that '12v remote on' leads may be better than 'power antenna' leads to connect to).

I took it to an auto-electrician earlier today and they seemed to think it may be that the connector is the wrong one and is going to try one without RCA leads instead. Is this likely to be the problem? (I thought RCA leads were fine to use with the rear amp though).

Any ideas welcome before my car turns into an even bigger money pit!!

Thanks for your input fellas. From trawling through other forums this seems quite a common problem ie no sound from the rear even when using correct adapters/wiring correctly. The auto-elec said they would try the PC9-401 adapter early next week (same as DefBlade is going to try I think) but if that doesn't work, I'll just have to replace the amp. The only clue I've come across is that on the Nexxia website, they say the PC9-404 is for HU output greater than 2.2V (less than this should use PC9-401), and my HU is 2.2V so it's borderline I guess. Still don't know how the crap radio reception fits in to all this though. Will let you know whether the new adapter works.
Nick

OK, I guess that rules out the voltage being an issue then. I'm planning on wiring the rears directly but if the new adapter doesn't work I would still like sub output - any suggestions for an amp? I've never gone down the amp/sub route before so excuse my ignorance - can I get something cheap to replace the stock amp (preferably something that fits in the same place and that the connector block fits straight in) and be compatible with the new pioneer HU and the stock sub?