Alistair Thomson
Forum Participant

According to the schematic you supplied of the 645 (the audio amps), there are only two speakers: Left and Right. The circuit uses one output transformer in each channel (T2 and T3), with no field coils, so any modern permanent magnet (as opposed to field coil) speaker with an impedance from 4 to 16 ohms will work. I suggest a diameter between 5″ and 12″ but of course the cabimet will have the correct size of cutout. There will have been an impedance specified in the original design, but in the absence of any more info you can try any of the above without causing any harm.