Altec 1650 Active equalizer

Hi there,

I recently got my first set of Altec gear which is a rare thing where i live (2 Altec1650 equalizers).
I looked at the manual from great plains audio and hooked up the equalizers at 600 ohm in / out and they work but i'm guessing the impedance is too low as they don't sound right and the output is too low.
I read that 600 was a sort of standard for telephone lines but it should be several 1000's of ohms higher to work with modern equipment.

Is there anyway i can connect it to modern equipment without any issues ?

Thanks in advance, :)

Wraith