Digital Music MX-8 (6 in, 8 out)
JL Cooper MSB 2+ (8x8)
JL Cooper Synapse (16x20)
Sycologic M-16 (default is 16x16, but you can add up to 32 more outputs)
I’ve used all the above over the years. I currently have a Sycologic M-16+16 out expander and am re-purchasing an MX-8 to handle my effects input overflow.
If you have a Mac (not sure if there is a comparable third party software for Windows) the old Emagic Unitor 8 MkII or AMT 8 work great and can be easily be setup with the free software found here: http://www.potm.org/software/Unitor/
I’ve got a Unitor 8 mk1 in the mail now to give me another 8 ins/outs, and you can stack up to 8 units if you need to expand your I/O.