The technical spezifikation says that bot have (for CD) 20- 20.00 Hz. Is the acoustic result really the same ?

All CD players will have a similar technical spec - I note the CDX, the first B&O CD player, is quoted as 3-20,000Hz!! The important bit will be the rest of the system - the BS9000 has a digital output which is better if you have Beolab 5s.

Do I think there will be much difference between the BS4 and BS9000 through Powerlink - not that I could hear.

So if you do not have BL5s with their optical input and you are trying to decide between the two BS4 versus BS9K look at range of sources they have ,,, and also consider the convenience of the BS9K's multiple CDs versus those CDs captured on to a 2GB SD card ,,, and the looks of the BS9K and its design & build quality versus the BS4 (every time I hear the click of the BS4's cover catching a CD I groan)