Sorry, but the crossover is still between amps and drivers. I just separated the two filters from being parallel to each other to being totally separate. Paul Joppa has designed a simple passive line level crossover for experimental purposes which could be adapted for use with Pi speakers. Unfortunately I can't advise on adding a suitable compensation and zobel network to this crossover. Check out the Bottlehead forum for more details. Paul's schematic may have been E-published on the latest Valve Magazine and is free for download. Valve can be accessed trough the Bottlehead Forum or the Bottlehead.com.