cabling a subwoofer to a NAD C356 integrated amp

I have the NAD integrated amp with A & B speaker terminals. I have a Gallo sub and Focal speakers. I'd like to have the Focals connected directly to the NAD, not through the Gallo. I'm wondering if the Gallo can be driven by the B speakers so I have A & B speakers driving together only for the B set I have my cables set for both right and left positive to go to positive in on Gallo, same for left. The gallo will not have any speakers attached, and I can turn it on and off at will. Any downsides to this?
For the B set I have my cables set for both right and left positive to go to positive in on Gallo, same for left.
The Gallo subs, or at least those that are presently listed on their website, have separate high level inputs for the two channels. So I think you are NOT saying that you would connect left and right positive to a single positive input of the sub, but rather to separate left and right positive inputs, respectively. And likewise for negative.

If so, and given that the Gallo subs are self-powered, there is no downside to what you are proposing. Also, I'll mention that you don't have to use heavy gauge speaker wire to connect to the sub, since you would be driving the high input impedance of the sub's amplifier and not the low impedance of its driver. Pretty much any gauge that is heavy enough to not be mechanically fragile should do.

The one thing you absolutely do NOT want to do is to connect left and right positive of the B output to a single input terminal on the Gallo, which could damage your amplifier. Although as I indicated I suspect that is not what you meant.

-- Al
the nad c356 has preamp outs and ins--would the poster be better served connecting the amp preout to the rca inputs on the sub (and rca outputs to to the amp prein)?
Hi Loomis,

I think you are referring to connecting the pre-outs to both the sub and to the inputs of the amp's power amp section, rather than connecting the pre-outs to the sub and the sub's line outs to the power amp section. If that is not the case let me know, although the considerations either way are somewhat similar.

That approach would probably work reasonably well. But I suspect the resulting sonics would be no better, and perhaps slightly worse, than connecting at speaker level. The main reasons being:

(a)The input impedance of the NAD's power amp section, at least for the C356BEE version, is specified as 20K in parallel with 1 nF (1000 pF). That is a large amount of input capacitance, which the output circuits of the preamp section would have to drive in combination with the added capacitance of the cables to the sub, as well as the input impedance of the sub itself. (The sub's input impedance is unspecified, but the input impedance of the line-level inputs of powered subs is often low enough to be an issue). The 80 ohm specified output impedance of the preamp section would seem to indicate that it could handle that reasonably well, but depending on the accuracy of that spec and on how it may vary as a function of frequency I wouldn't assume that the resulting effects on the signal path to the main speakers would be negligible.

(b)I suspect that the line-level connection approach would stand a somewhat greater chance of resulting in hum or other noise issues than the speaker-level approach, although chances are it would not be a problem either way.

(c)Although some folks would disagree, a philosophy espoused by REL among others is that it is preferable for the signals provided to the sub and to the main speakers to both reflect the sonic effects of the main power amp.

Best regards,
-- Al
I greatly appreciate input from all respondents. The guidance was most beneficial.

I recently learned something about the Gallo that is quite important and finalized the cabling. Not only is there a low pass filter that can be changed for crossover point, there is also a high pass filter fixed at 80 Hz. The Focal speakers have a (to me) loose lower bass I often heard with ported designs that thought a bigger orfice meant more bass. The speaker has two ports, both large diameter, one downward firing and the other front firing. When I connected them to the Gallo the bass tightened up and I had effective resolution down to 25Hz per the test tones I downloaded from the Focal site. I thought it was the sub doing its job supplying lower bass, but didn't ask myself where the boomy bass went. Well, now I know. Given this feature I'd be stupid to electrically separate the sub from the speakers. I'm going to unhook the sub and run the speakers straight from the amp, listen to the tone sweeps a few times, then hook the sub back in and do the same listen. It will be interesting to note the difference.
Good! That's a comparison that is well worth doing. As you realize, whether or not it is preferable to high pass the signals provided to the main speakers involves a system dependent tradeoff between how well the speakers and the power amp can handle deep bass content, vs. the side effects that may result from introduction of the sub's high pass filter network and output driver and other circuitry into the main signal path.

There doesn't appear to be a spec on the output impedance of the sub's line level outputs, and the sonic quality of its high pass filter circuitry and output driver circuitry certainly isn't quantifiable, so comparing the two approaches is pretty much the only way to tell. The only suggestion I would make, which you are most likely already planning to do, is to be sure to assess the differences between the two approaches with respect to all audible characteristics, not just the deep bass response. For one thing, it's quite conceivable that the side effects of introducing the sub into the main signal path would be evident mainly in the treble region.

-- Al