Perp,
I sold all these products back in their heyday. To distill the various advice to clear instructions...
1. Your amp must have a "tape loop", "external processor loop", or pre-out/main-in jumpers in order to incorporate an EQ in the signal path.
2. The amp that drives the 901s should not drive any other pair...speaker B can only be another pair of the same generation 901s!
3. If your amp has only one set of tape or preamp outputs, use a pair of Y connectors to double (1M>2F), or use duplicate outputs if it has them, to run into a second amplifier...preferably an integrated, to set relative volume. Choose any line level input. EQ with your graphic if desired and if amp 2 has a loop too.
4. Early generation 901s (SeriesI and II) had MUCH more severe EQ characteristics than all later models, so the danger of damage to another speaker is greater with those. No graphic EQ is equipped with the boost to flatten the early ones' response...at 30Hz, they had over 20dB boost!