yes line level.
Most amps require a MAXIMUM signal of 200mV to 2000mV. Your spec sheet should tell you what your sensitivities are. You need the same gain in each filter/amp/speaker combo. If all your speaker units have the same sensitivity and all you amps the same gain then you can manage without gain adjust on the filter outputs. Otherwise add a buffered gain adjust.
The voltage that your filter opamps run at could be +-15Vdc. The maximum signal through the filter string and output will be approx (15+15)/3 Vrms i.e. about 10Vrms into a high impedance and somewhat lower into lower loads. This will easily drive any unbalanced amp input with an impedance >=10k.
I guess I phrased the question this way, because I have been thinking of getting an integrated amp that may or may not have pre-out/main-in jacks. As a result, I was trying to figure out where to stick the crossover.
But it doesn't say where you get the input signal for the XO. In a case where you have no pre/power inputs on a receiver or integrated, speaker level inputs can be used (almost every plate amp has them). This is usually considered a compromise, but there is a school of thot that has the SS sub amplifier feed from the output transformers (ie speaker level) of a SET to get better integration of the sonics of the woofer & the speakers on the top.
Yes but you still need power amps after the electronic crossover. Besides, when the signal is taken from speaker level it has to be greatly attenuated. Even a sub plate amp has a power amp built in. Perhaps the originator of the thread could more fully describe what he was trying to do.
If indeed you are merely trying to make an electronic crossover for a sub woofer, you can indeed use the speaker level outputs of your source. You just have to attenuate the signal to line level. Use resistive dividers to attenuate the signal about 10:1. Then run this through the sub-woofer filter, while running the other speakers full range, and after the sub-woofer filter, to a sub power amp.