I bought a HU for my dads A4 (2000) as his disply hadn't worked since he got it. Then it just stopped working.
The new unit supplied from a 1999 A4 comes up with BOSE on the display and seems to work ok but I get nothing through the front speakers even when I use the fader. I tried it in my '98 A4 and it does the same thing. I read somewhere the BOSE is slightly different from the standard unit. Both stereo cassete headunits look identical.