I'm doing exactly what you trying to do. I use a Parasound 21 for the woofers and a tube amp (set 300B's) for the tweeter and midrange. I looked at your preamp outputs. I run my Parasound with XLR outputs from streamer/Dac directly to the Parasound and RCA outputs from streamer to my set amp. My streamer /Dac does not have an output selector so both outputs work at the same time. You will have to figure out if outputs from your preamp can be run simultaneously. Otherwise you will need a splitter. The gain from the XLR out will be twice what the RCA output is, so if your stuck using a splitter use XLR. Also, you are correct as the gain control on the Parasound can be used to set the woofer volume to match the tube amp output. I use a program called REW and microphone to adjust the output/volume levels on the Parasound to match the tube amp volume. But that's another "rabbit hole" No messing with the crossovers-Parasound to woofers and tubes to tweeter/midrange speaker posts. Sounds sweet.
