The RPI 3 seems to be fast enough for digital low-highpass filtering of sound. Can I connect two hifiberry dac+ cards and send divide left and right channels through high and lowpass filters direct to my poweramps?
Sounds like you're trying to build a crossover?
You can't, to the best of my knowledge, stack multiple HiFiBerry DACs for more outputs. Their help pages have an outline of which pins are used for what.
Personally, I'd go for an outboard crossover unit of some kind. It'll cost more, but stand a higher chance of success when you're already fighting against the Pi's limited capabilities and godawful audio output.
I found a solution for filters on http://www.instructables.com/id/Raspberry-Pi-Music-Server-With-Built-in-Crossover-/?ALLSTEPS, but mr.J.Ruby uses the hdmi output and uses the receiver to decode to analog. But I am sure the rpi3 is more than fast and can be used for filtering. I like the hifiberry dac+ pro card it sounds perfect. Look at this article: www.instructables.com/id/Raspberry-Pi-Music-Server-With-Built-in-Crossover-/?ALLSTEPS