Welcome to Beoworld and thankyou for supporting the site with a Silver membership.
Personally I would keep the CDX and the Redlines and buy a suitable 1980's Beomaster like the 2400-2 as you would have a great system. The Beocenter is undoubdetly the weak link.
If you are determined to sell you may have more luck splitting them up and selling seperately. Not many people will need all three items at once, especially if they already have some B&O.
Of the three items the CDX is far and away the most sought after and must be newer than 1978 as there were no CD players available then! It could fetch £100 - £150 on it's own if it is in mint condition.
The redlines could fetch anything from £30-70 depending on condition and the Beocentre 4000 doesn't get much love as it was made in Japan rather than Denmark and hardly anyone uses cassettes any more!
Simon