Using an attenuator solely would be passively biamping and yes would allow you to adjust the gain between two dissimilar amps. However, IME/IMS active biamping with an external crossover is so much better sounding than passively biamping. I say this even if you weigh the added expense and space of the crossover. I have been actively biamping for close to a month and it has been and significant upgrade to my system (I had tried passive biamping with mild improvement.) The active crossover also allows you to adjust gain. But just as important, it directs only those frequencies that you send to the speaker drivers. eg, my woofer amps only see frequencies below the crossover I specified ( you need to find out from your speaker manufacturer what the crossover frequency and what order crossover they designed for the passive crossover.) Thus the woofer amps are much more efficient because they need only be concerned with those frequencies for its attendant drivers. It is the same situation as one would use a subwoofer in a HT setup.
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