How to split stereo into stereo plus sub out?

Consider a budget system that plays HD music files from a PC using either USB or Aptx HD Bluetooth. If I am using a DAC similar to the SMSL M400 or Topping D90SE and it is connected to a Yamaha P7000S amp which powers a L and R speaker, what is the best way to add dual powered subwoofers?
I own two SB-2000 subwoofers, but I am not sure how to connect them into the system above because the amp doesn't have any sub-out or pre- outputs of any type.

I have not purchased the SMSL M400 yet, but none of the devices in its class have subwoofer outputs (as far as I have seen).
Without spending a lot of money, how should I connect my two powered subwoofers into this system? Thank you.