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.
lowoverdrive
If your speakers have "line level" outputs, without having to modify anything, give that a try. I had never heard "non-active" speakers having line level outputs before, learned something new, thanks. I think my idea will work just fine too, but yes, you will need more low-level cabling. It's my opinion that low level inputs work better for MY subs as far as sound goes. Many REL "experts" say "hi level" input are more desirable than "low level". I've tried it both ways and low level from the preamp outs or power amp ins, win. What I meant by don't mess with the high level outputs by adding resistors is; you could cause a short and damage your amplifier or hurt someone... 
@lowoverdrive,
Since the SMSL M400 has both xlr and rca outputs, the easiest way to connect the subwoofers is via rca cable, and connect xlr output to Yamaha P7000S.
@imhififan  - thank you. I tried it, and it does work. It is very simple. This combination (SMSL M400 & Yamaha P7000S) seems surprisingly good.