Best way to integrate a subwoofer into Luxman Preamp + L-509X Integrated system?

I have the Luxman L-509X and it is an incredible sounding integrated amp, particularly once you give it a little volume. And when you get to higher volumes, it really starts to sing. I’m currently running dual SVS subs out of the pre-out on the back of the L-509X and everything works great (setting crossovers at the subs). Main speakers are Klipsch La Scala IIs.

At lower volumes when I am listening at night, the L-509X loses a little bit of its sparkle and depth, and I’ve found I can replace this by using my vintage Luxman CL40 as a preamp, and using the L-509X’s "Separate" setting (basically HT Bypass).

This bypasses the preamp and volume control in the L-509X, and allows me to use the CL40 into the "Main In" input as a tube preamp. At lower levels on jazz and acoustic music particularly, this adds incredible dimension and sparkle to the music.

Really a "best of both worlds" solution here, because the CL40 adds a good bit of distortion and grain at higher levels, and the L-509X excels at clarity, power, and dynamics at higher levels.

But the problem I’m running into is that if I’m running the subs out of the pre-outs on the L-509X, they are volume-independent from the CL40 preamp volume. So I have to manually adjust the subwoofer volume on the integrated volumen knob to roughly match where the volume level is on the CL40.

Running the subs from the CL40 Output 2 is not an option as it removes the subs from the L-509X for daytime listening and movie content, etc.

Running an alternate out from my Hegel HD30 DAC to the subs could be an option, but I’ve run out of outputs since I’m using the balanced XLRs out to the L-509X and the unbalanced RCAs to the CL40.

Is there any solution outside of using some post-analog DSP such as a Mini-DSP on the outputs that can give me subwoofer control tied to the main volume control regardless of whether I’m using the CL40 preamp or the L-509X as a standalone integrated?

Worst case scenario is I’m turning two volume knobs for matching between mains and subs when I’m using the CL40, but that isn’t ideal.