I would sell on eBay. This is an interesting system - although it is actually not made by B&O but is made in Japan! Parts are a problem - particularly for the amplifier. Your problem is that no-one uses tapes any more! I would expect it to sell though as it is a good party machine - over 2 hours of continuous music possible!!
I am afraid the X25s are not one of B&Os finest efforts. Some people might like a pair for a link room and like the grey finish though.
Price - well i have seen 4000s sell for £5 and I have seen £150. Rather depends on good advertising and a bit of luck! I cannot remember but either the 4000 or the 2200 (record deck built in instead of second tape) was in the Kylie Minogue video of Lucky in Love!!!