Helping my brother in law out - he's got a JBL passive subwoofer for home theater. It's got 1 set each of left/right inputs & left/right outputs, like this:
Input
L+  L-
R+ R-

Output
L+  L-
R+ R-

I've got a NEC M-50 single-channel amp -


Wondering if, or how, I can or should hook it up to power the sub. The receiver he's using has a pre-amp out for subwoofer, and the amp has a direct input, so that's easy enough. The amp has A and B speaker outputs, but they cannot be used at the same time (A overrides B if both are turned on)

Should I run the inputs in series or parallel, or just run the power to one set of inputs (either L or R)?

What's the impedance of the sub?
Assuming it's 8 ohms, you can wire up both speakers of the sub in parallel because the amp says it's good for 4 ohm loads




right on the back plate.  
The wiring is red to red to red,   and black to black to black.   This is PARALLEL wiring.  The speakers are wired side by side,




sharing the same connections.
Simple.
Now, if the sub is 4 ohms per side,  you have to wire in series instead.
Red from the amp goes to the first red on the sub.  The first black on the sub goes to the second red on the sub.




The second black on the sub goes to the black connection on the amp.   This is SERIES wiring.  One after the other.




It will turn two 4 ohm loads into one 8 ohm load.  
However, that little amp isn't all that powerful.  Don't expect it to shake the roof off.   I think I see that its power consumption is listed as



100 watts, which implies no more than about 60 watts for the speakers.    Probably 50, given the model number.





I like to run about 500 watts per channel into a sub.  Headroom is good.  
CJ
 

Thanks cmj - that little amp's a mini-monster monoblock (specs). Heavy too. 50w into 8 ohms, 100w into 4 ohms. Should drive that speaker well enough for their small basement home theater.

I'm not sure of the sub's impedance at the moment - I have to wait until I bring them the amp and take another look at it. It is a single 10" speaker, with inputs for both l/r. my guess is it was part of a sub/satellite set at one point. Will the fact that I'm sending signals from a single channel to both l&r inputs be a problem? I suppose I can wire it up & try it at low volume and listen for clipping as I turn it up, to see if it'll be okay.

The parallel option sounds good. I think I'm going to try hooking up just to L or R first and listen, then try them in parallel to see the difference. If the speaker impedance is lower ( which I doubt ) I'll do the math and see if it drops the ohms below 4 before trying it out.

I tried talking him into buying a powered sub, and got "meh" as an answer. I've got a powered 12" at home I've been using for years. He always comments how good it sounds. I guess you get whatcha pay for.

You are correct.  Most passive subs like you describe aren't going to handle much more than 100w.

It won't be an issue.  The sub is a dual voice coil and I do what you are describing all of the time.  In fact, my home subs are quad voice coil and I run each with a single amp channel.  Multiple voice coils just give the end user wiring options...If the sub is dual 4 ohm and you wire it in parallel, then you may send your amp into protection mode.  In the rare case, you will let the magic smoke out of the amp.

Do not drive one coil of a dual voice coil sub.  Not only is your power handling cut in half, you are drastically changing the electrical side of the Theile/Small parameters of the driver.  Get a multi-meter and test the impedance of each input of the sub.  An 8 ohm voice coil should read about 7 ohms of DC resistance.  If you want to leave this as a long term solution, you should also remove the driver and permanently remove the passive crossover from the sub.  Or at least bypass it via re-wiring.

Thanks again to both of you. I'll let you know how it sounds after I wire it up, probably this weekend.

-D