Thank you for the response.
Using a pc as a loudspeaker crossover is looking complicated. I'll keep reading and learning and maybe try it when I get enough knowledge.
Can anyone share their experience with a pc crossover?
Dan
So I've been using my computer as a crossover with JRiver for 10+ years. I have had no luck whatsoever trying to sync multiple USB DACs, and I wouldn't encourage anyone to try to do that unless their DAC manufacturer specifically advertises support for that (like some Motu DACs do).
So unless your existing DAC explicitly supports syncing to another DAC's clock, I would recommend that you get a hold of a single multichannel DAC that has enough channels for your system. I'm currently using an eight channel Topping DM-7 and am very happy with it, but there are numerous other multichannel DAC offerings out there from Okto, Focusrite, Steinberg, etc.. Those interfaces will let you use JRiver's DSP to it's fullest extent.
Alternatively, MiniDSP will also sell you competitively priced programmable "crossovers in a box", like the MiniDSP Flex 8. A MiniDSP does all the crossover work in the box so you wouldn't be using JRiver's DSP chain. I currently use a MiniDSP in parallel with JRiver, but the DSP available in the MiniDSP is less robust than what JRiver offers (It's is fine if you only need a bit of DSP but doesn't have enough filters/taps for more complicated setups). I keep the MiniDSP around as a fallback for web audio and other things I can't route through JRiver, but if you just need a crossover and a few tweaks you could use the MiniDSP as your main crossover/DAC