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
You do not need a splitter. Connect the LOC to whichever speaker outs you want. At 20k (or more) the LOC presents near zero load to the amp. It will not affect anything. Also the bass being used to drive the subs is so low it is not even stereo. So you could if you wanted connect all the subs to just one channel.  

You are right to be thinking about impedance however at 20k ohms or even higher it simply is not a factor.
If subs have speaker level inputs you can use the same speaker binding post for both main speakers and subs. The high impedance of the sub amps will prevent current draw from amplifier.
SVS SB-2000 do not have speaker level input, line level in only.
It sounds like you will be using the DAC as the volume control.
If you are using RCA cables from the DAC to the Yamaha amp then the easiest way to deal with this is to buy an RCA Y-adapter for left and right channels. Plug Y-adapter into DAC and run RCA cables to everything.

https://www.amazon.com/Mediabridge-ULTRA-RCA-Y-Adapter-Inches/dp/B005DL2OVE/ref=psdc_172544_t2_B004E...
Are you using the XLR's for the amplifier inputs? If so, getting a 1/4" TRS phone plug adapter to RCA's(if that's what your running to the subs) would be simple hookup. XLR's and 1/4" TRS inputs on that amp are in parallel, so both will see the change in output from your preamp or DAC. Don't mess with high level outputs of the amp and resistors, you are asking for trouble. Y's will add noise.
Connect the speakers to the speaker outs on the sub. 
My speakers do have line out RCA's which the manual describes like this:

Line Level Outputs
These outputs are used in 2-channel applications to high pass the signal being
sent to the loudspeaker amplifier. The line level outputs feature a fixed 80 Hz
12 dB/octave high pass filter.
My subs only have line level inputs as well. There are no speaker inputs.
As far as I understand, this option would require running speaker cables back to my amp (from the subs) and then from my amp to the speakers. That will add substantial length to my total speaker wire. Is that a problem? I thought it was best to keep speaker wire length as short as practical.