There is plenty of AV9000 around for sale.
It just takes 2 people and the willingness to actually go and pick it up besides paying the seller. They are heavy.
The centerspeaker is default and always the AV9000's first choice when ever its turned on.
Unless the thing you are watching are musically content like a concert dvd of your favorite artist the centerspeaker works quite well.
Stereo sound without Beolabs? Yes it can be done.
The way I would be doing it is to bring the Beosystem 6500 back in the living room and place the Beovox's on either side of the AV9000.
I would then be able to use AUX and get the sound in stereo that way. I'm still connecting the Beosystem 6500 via 1611 converter to AV9000.
The AV9000 is based on the MX6000 in the electronics but owerall you don't lose anything other than your favorite design really. If you do not plan to buy at least front speakers then a MX7000/8000 is the better buy. Also it might be better for linking.
I can only compare to my Beovision LX5500 but as it is the LX wins on the sound. Picture is a tie. The LX wins for most value for money and reliability.