How to split stereo into stereo plus sub out?

Two ways, both equally good, one free the other maybe $20 tops.  

The free way is if your powered subs have speaker level inputs. Simply connect the subs to the amp speaker outputs.    

The $20 way is to use what is called a Line Out Converter. The LOC is nothing more than a couple resistors, typically 20k to 50k Ohm, that drop speaker level voltage down to line level. If you do this buy the cheapest one you can find, unless you care about how it looks because that is all you are paying for, a better looking box and RCA.  

The third option for about $5 you probably won't want to do as it calls for some soldering. Basically you build your own LOC using your own resistors and connectors. Here is the interweb's best tutorial on the subject, in case you really want to understand what is going on.  https://www.epanorama.net/circuits/speaker_to_line.html/  

Here is my system page where you can see a photo of the LOC mod that I did to my Melody integrated following the info in the epanorama article https://systems.audiogon.com/systems/8367

Here is my system page where you can see a photo of the LOC mod that I did ...

That's an amazing looking system! Wow.
In regard to the free way and the $20 way, my amp only has outputs for 2 speakers. I'm using banana plugs and good quality speaker wire.

I assume I need to make some splitter cables so I can connect both the existing speakers and the LOC to the same output from my amp.

I don't know enough to ask the right question, but it seems that I should be concerned about impedance or other similar issues if I put a splitter in the system. Is that true?

For free.

Connect the speakers to the speaker outs on the sub.  

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.

@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.

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.