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If you are summing the low-passed outputs from several active crossovers, you won't get any crosstalk, as the signals are buffered by the opamps used in the active xover stages.
If you want you could always add a simple unity gain buffer first to each signal THEN resistively sum. Or you could be flash and introduce a summation using active circuitry.
I seem to recall you are using Rod's P09 boards? If so, I've summed both low passed channels on mine. I added the summation after the final buffer stage (Which I kept in to provide a bit more gain range for the low-passed signal - my subamp didn't have enough gain compared to the the amps doing the highs). Basically I replaced the output 100ohm resistors with 22k resistors and tied both ends together before passing through the output cap. This works fine.
Generally you can just sum two signals in the same way - just run them through 22k or so resistors with their ends tied together. The crosstalk you get is then determined by the output impedance of the pre-amp which is usually low enough not to be an issue.
Hope this helps. Rod may have some other ideas if you've used his boards.
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