I have two different aggregate device setups.
The smaller one consists of a MOTU 828MKIII Hybrid and a MOTU 16A. This pairing produces an aggregate device with 64 ins and 62 outs. The MOTU 828 is the clock source, everything is set to 48khz. This device operates without issue in Cubase at a buffer of 256, 32bit float processing, ASIO set to normal, boosted audio priority, and a disk preload of 2 seconds. It also works without issue in my other DAWs (Logic, Studio One, Bitwig, etc.)
The larger aggregate device adds one more interface to the above and consists of the MOTU 828MKIII, the MOTU 16A, and the Expert Sleeper ES8. This creates a device with 76 ins and 78 outs. As before, the MOTU 828 is the clock source, everything is set to 48khz. This device runs fine in my other DAWs, but Cubase will not work with it. If I try to use this device with Cubase, the CPU meter overload indicator is constantly on and cannot be clicked to reset. However, none of the activity meters show any activity.
The program GUI is responsive, but no soft synths or wav files will play; there simply is no audio produced by the system when I use the larger aggregate device and and that indicator is active.
However, if I go into Audio Midi Set Up (with Cubase still running) and remove the ES-8 from the aggregate device (making it 64in/62out again), the CPU overload indicator immediately turns off and everything plays normally.
This leads me to believe that there is an I/O limit on aggregate devices.
Thanks for reading this far!
EDIT: SYSTEM SPECS: iMac Pro 10 core, 64gb, macOS Big Sur 11.6.5, CB Pro 12.0