How to split stereo into stereo plus sub out?

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.

@millercarbon - thank you for the good explanation.

@bryhifi

Thank you.
> SVS SB-2000 do not have speaker level input, line level in only.
Correct.
> It sounds like you will be using the DAC as the volume control.
I can use either the DAC or PC as the volume control. I can also set a general volume level with the amp. At the moment my DAC is set to fixed volume level in its settings and I'm adjusting volume with the PC.

> 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.

Yes, that will certainly work for me. Thanks for the link.

@fiesta - thanks

> Y's will add noise.
OK.
> Are you using the XLR's for the amplifier inputs?
Yes.

This amp has:
INPUT jacks (Channels A, B). Connections available:
- XLR- 1/4 TRS

OUTPUT jacks (Channels A, B). Connections available:
- Neutrik NL4FC Speakon output connectors,
- 5-way binding post output jacks,
- 1/4 TRS

> 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.

I like that idea. I'll try it.
Let me make sure I understand. I'll continue connecting XLR out on DAC to the XLR inputs on my amp. (The amp to speaker connections remain unchanged.)I will add two 1/4" TRS to RCA adapters and connect the TRS outputs on the amp to a LOC and then to my subwoofers via RCA cables.
Is that correct?

> Don't mess with high level outputs of the amp and resistors, you are asking for trouble.

Can you explain what you mean? I didn't follow that.