The 9000 was probably the best though to be honest, a 8004 or even a 5000 can possibly match it in performance. That is because the clever trick of the 9000 was calibrating the tapes and just after it was introduced, the tape manufacturers standardised them much more closely. Therefore the best method is to pick a tape and set up the cassette deck for that tape. The 5000 in particular can be tuned very well and is very cheap these days. I would not advise spending a lot on a cassette machine as one can get a 5000 for about £30 and it will produce as good a result as almost anything.
If you have an 8000 system, then the 8004 or 9000 are worth getting - but don't pay too much!
They will have to connect to an Aux socket on your present equipment. They will not work directly with Beolab speakers.