Line level converters are super simple to build even with super basic soldering skills. Get a little plastic box off Amazon, a set of speaker binding posts (4 total), a set of female rca connectors (2ea), 1K 1/4 watt resistor (2ea) and 10K 1/4w resistor (2ea) and a pinch of wire. Drill your holes in the case, mount your rca and binding posts, then solder the 1K resistor across ground to + on the rca. Next wire the - binding post to the ground rca, then finally connect the 10K resistor from + binding post to + rca connector. Taaadaaa, you built a fantastic line level converters. Basic binding posts and rca will give great results, but upgrading to WBT or my favorite being KLE connectors, you’ll get even better performance. All in all, basic setup should be $30 and an hour of time. When I ran converters, I even did away with the binding posts and just hot glued the speaker cables into the case and wired straight to the rca.
Also on these, since it’s all 1/4w resistors, there is no extra power draw from your amplifier so unless you’re running a flea watt amp, there’s no worries about headroom. And if you need to lower the signal even further, just up the value of the + leg resistor from 10K to 15K, 20K etc until you get the value where your sub blends using 40-50% volume. I tried loading a photo but it's not wanting to cooperate. Sorry.
Hope this helps,
-Lloyd