Hi,
I searched high and low for a replacement Bose amp for my 04 2.0 Tdi and came across the same problem, nobody could/would confirm that a similar Bose amp from and A4/A6 with a similar number would be interchangeable. I rang Audi and spoke with an 'expert', he stated that you had to have the same one to the one you are removing. I searched high and low in the UK and could only find a replacement at £150+ as you have found, my expert at Audi advised new is £400+!!
I ended up locating one on Ebay in Germany and made an offer, which was accepted and cost me just over £100 delivered to the UK. He did have more than one when I bought mine a few months ago!
Just need to find a working (no volume problem) Concert for it now.
I take it your amp got wet behind the rear trim panel, causing it to melt out, great Audi design to place an amp under the rear washer pipe joint!!
Good luck.
Steve.