How to split stereo into stereo plus sub out?

@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.