Subwoofer connection conundrum

Need some help, as I'm in over my head. I've got a new NAD C 298 power amp -- a gift from my wife! -- that uses the Purifi Eigentakt class-D amplifier module, or a modified version thereof. As I understand it, and that's not so far at all, I should not hook up a subwoofer using the speaker-level outputs because of something in the design of the circuit. (Or something.)

I'm wanting to get a pair of subs that I'd place inside my speakers -- KEF LS50s -- and had been thinking of a pair of SVS SB-1000s, as they are about the right size and price. I was planning to use the SVS wireless adaptor, until I learned about latency. So, finally, to my question:

My preamp, a Wyred 4 Sound STP-SE, is connected to the NAD with a run of balanced interconnects. (The amp is between the speakers, with the rest of the equipment on a side wall.) Am I assuming correctly that I'd need to run balanced to the subs, too? If so, can anyone recommend a pair of modestly priced, modestly sized subs with XLR inputs? (The W4S has another set of balanced outs, so that's not a problem.)

Finally, could I run single-ended from the pre to the subs, or would that be a problem because I'm running balanced to the power amp? I understand that there'd be the possibility of noise, but I'm not certain if there's some other reason why it would not be wise to run balanced to the power amp and single-ended to the subs. If that would be OK, it would obviously open up many, many more possibilities for subs, as my searching so far has found few with XLR inputs that aren't giants.

Thanks very much for any help you good folk can provide on this.

-- Howard
You can run the subs unbalanced RCA no problem. You can also run the off the speaker outs on the power amps, but in this case you will need a Line Out Converter to drop the voltage down from speaker level to line level. Sub interconnects do not need to be high quality. Which way you do it will have as much to do with how much you want to spend and how you want it to look. In terms of performance there will be no difference.  

What will make a difference is where you put the subs. They do not need to go between the speakers. In fact they will work a lot better with one in each corner, but each one a different distance from the corner. This alone will be far greater a difference than anything you can do in hooking them up. Do a search for Distributed Bass Array. 
+1 to everything from @millercarbon on this question.
  • Speaker-level input is an unnecessary complication.
  • Unbalanced runs typically are fine for a sub.
  • With subs, positioning is critical. Asymmetry will help give smoother response.

Here's what's got me nervous about using the speaker outputs:

The C 298 manual says the following:

  • The blue terminals must never be connected to ground (earth).

  • Never connect the blue terminals together or to any common ground


  • Do not connect the output of this amplifier to any headphone adapter,

    speaker switch or any device that uses common ground for left and right channels.

    I'm nobody's electrical engineer -- far from it -- but doing a bit of reading around has made me worry about the wisdom of trying to hook up a sub off the speaker outputs of this amp that's employing some newfangled technology that I can barely pronounce, let alone understand. I'd really hate to blow up my nice gift after just, like, two weeks.

    And I've been liking the way it sounds, too!

    -- Howard

Post removed 
Perhaps Miller Carbon is not familiar with some of the new class D technologies. For the edification of someone who might run across this thread at some point down the line, here's a post from our friends at REL:

Here’s where Class D amps get weird (not all, but about 90% exhibit the anomaly I will describe next). The Black speaker terminals on the rear of your amp traditionally, and by electrical convention, mean Earth Ground. Zero ohms, zero Hz. Ground. But with most Class D amps, that Black-colored speaker terminal* is no longer ground. Worse, it actually references to a positive voltage that is significant. We consistently measure 10-15 volts ABOVE ground. In practical terms, if your REL were mistakenly connected to a Class D amp or receiver the input of the REL would eventually be burned out because it would be pulling 15 volts through a circuit designed to see zero voltage. Over some period of time—months or a few years, this unwelcome voltage will burn out the front end of your REL.

The Solution:

Fortunately, the solution turns out to be both easy and inexpensive—potentially no cost difference at all if your system is theater-based.

  1. On the REL High Level cable go ahead and connect up the Red (R+) and the Yellow (L+) leads as usual. “Float” the Black wire (do not connect it to anything), in fact, snip off the portion that is pre-stripped and wrap it in electrical tape to avoid accidental shorting out later on.
  2. Next, connect a long RCA-RCA interconnect from either an unused input on a preamp or receiver to the REL’s .1/LFE input. Do this even if you have no interest in theater, you are creating an audio ground, nothing more. This is true for 2-channel systems, we are not using the .1 LFE input for anything more than to produce ground.
  3. If you are running this in a theater system, you’re done; run your usual .1/LFE cable and float the black wire and you’re done.

*One final call out, NAD recently introduced two new integrated amplifiers. We would like to commend them for specifically choosing Blue for their “Non-Hot” speaker terminal. Rather than Red/Black, the Red/Blue scheme gets people wondering why the color is different.

Along with our friends at Peachtree Audio, the NAD folks also added an old-fashioned knurled chassis ground bolt which makes it even easier than we described to obtain true ground on this style of amp. We salute these customer-centric companies.

In the case of a chassis ground bolt like this, please DO connect the Black wire to this bolt.

There ya go, MC. Run that up your flagpole and salute it!

-- Howard