I bought the old original 2X4 right after they first went on sale many years ago. That 2X4 was superseded by the 2X4 HD, and the now newer DDRC-24. The DDRC-24 is supposedly a 2X4 HD upgraded with Dirac Live software for room correction.
Yup, I use the unbalanced RCA jacks on the 4X10 HD. It has one pair of RCA inputs for the PL2000 pre, and four pair of RCA outputs which will drive four amps.
It can also operate as a preamp if I unlock the Master Volume Control in the software. That single knob on the front has multiple functions.
I keep the the MVC locked on 0 dB unity gain, and use the PL preamp volume control.
Truth is, the 4X10 HD is old tech. Check out the DDRC-88A with Dirac and optional Dirac Bass Management. It's not available due to parts shortages. Look at the one that's been on eBay for several weeks
Minidsp has some interesting stuff. Unfortunately, they have no good offerings right now for what I want other than running dual 2X4 HD to drive four amps via RCA. Yes, one can easily run multiple MiniDsp products in one system.