What is your goal, essentially?
If I understand you right, you'd like to run a 7ch surround setup with passive speakers and lack amplification channels, so added a power amp, but due to lack of preouts use line out to connect.
But if the goal "just" is to power seven passive speakers in surround (plus potentially an active sub), then why not just get a second hand AVR?
If you don't need Atmos or the latest and greatest HDMI, there are usually many cheap options. I'm not in the US, but just yesterday there was s Harman Kardon AVR630 posted in my area for just 38$. It has HDMI, 7x100W at 8 Ohms IIRC, and even 8ch direct in and 8ch preout.
I would definitely have bought it, had it not been so "unshipable", if nothing else then just as a temporary power amp (it's got RS232 and RJ45 for a remote control protocol).