Your concert does have pre-outs they are just on a non standard connector, not RCA's - which is why you have to use the adaptor when fitting an aftermarket HU.
Regarding the Becker then according to the spec it has pre-outs as well, both front and rear and an adjustable sub. Thought it would as Becker are a really decent brand.
AUDIO
Sound control: source-separated treble/base control, adjustable balance and fader, loudness
Music output 4 x 30 watts
RMS output 4 x 18 watts
Pre-out 4 channels plus subwoofer/centre speaker (adjustable).
Looking more closely at the pics, the Becker actually uses the same non-standard mini-ISO connector that the original Audi unit & Blaupunkt units use, so you don't actually need the adapter at all. Just plug the Audi connector into the the long thin socket at the top and away you go. The problem you will face is with the iPOD connection which uses a different part of the same connector i.e. the CD changer input part. Can't see from the photo's what connector the iPOD adapter gives you, but I imagine it's the same as the RCA adapter you already have, just with the other end wired, and nothing in the other 2 sections?
If this is the case you'll need to extract the pins from the Audi socket (the ones going to your CD changer lead) and insert the pins from the iPOD adapter.
Can you post a pic of the IPOD adapter lead, I know it probably has RCA's on it, but is there anything else?