Reddit reviews Qianson 200W Pure Bass Subwoofer Crossover Second-Order Crossover Frequency Distributor Divider 2 Way Speaker Audio Crossover Filter

I mean, that is what a crossover does. You dont have to remove the dac or bypass it at all. You just want a low pass filter from the sounds of it. Something like this perhaps? Not sure of your whole setup though, if you put a low pass on the speaker with something like this which is a cross over, just for low end. No personal experience with either of those items specifically btw. Good luck on your adventure!

If the amp has no built-in lowpass filters, then all you'll get is a full range signal from the speaker outs you want to drive the sub from. Otherwise you could have used a miniDSP to split the signal before the amp and send the low frequencies only to a sub. There are even simple passive units you can put on the low level before any amplification, like https://www.amazon.com/FMOD-Crossover-Pair-Low-Pass/dp/B0006N41EM

But if you want to use that amp you do have to put the low pass filter on the speaker cables but before the sub (or inside the sub, by all means) - https://www.amazon.com/Qianson-Subwoofer-Crossover-Second-order-Distributor/dp/B01IZ6OBAA something along those lines.

Alternatively, use your amp for the speakers in 2 channel mode and add something like a Crown XLS https://www.crownaudio.com/en/products/xls-1002 - the reason I mention that is that those are very smart, can be bridged, and they have built in low- and high pass filter options. So you can send a full range signal to the amp, and it will cut out the highs itself. That way all you have to do is split the input after the pre-amp (assuming you have a pre-amp) and send the same signal to both amps.