Hi Gez,
I have just replaced the girlfriends TT's Bose amplified sub HU with a Pioneer double Din system. Pretty much Stevie Wonder could do it. As mentioned, you can buy the wiring kits that are literally plug and play... If you have steering controls you can get these to work with limited functionality too (example... "Track" and "volume" buttons work but "source" does not). Connects 2 is a brand of wiring looms/adapters I have used with no issues.
As far as the sub amplification goes the wiring feeds it all through correctly and you merely have to connect the red and white screened wires in the correct holes in the rear of the new HU.
Also (and apologies if I offend any purists out there) IMHO the new unit sounds so, so much better than the Audi unit.
Happy wiring!
