Jurriaan:
The AU-X501 model goes back to the late 80's. You might notice the newer design of the Sansui logo on the amp, which happened in 1987. The amp has a rating of 80 Watts and its in the middle of the product line, with the AU-X901 at the top (130 Watts), followed by the AU-X701 (100W), and then after your model, the AU-X301 at 65 W.
As I mentioned in another post to Tony regarding his AU-X901, I have the Sansui brochure on these models which explains the whole design philosophy.