I'll try to make it short.
Problem: I want to add a subwoofer to my 2-ch audio setup. The power behind all that is an integrated that has no sub pre-out. To make the story even more complicated, The only set of pre-outs I have is a stage pre-out so if I remove the jumpers there is no sound that will be fed to the input so the result is no sound at all.
NADs generally lend themselves quite well to these situations thanks to that removable pre-out/main-in link.
Depending on the sub itself, you may have several options. All of which assume right and left line-level inputs at least. If it only has one line-level input, it can get a bit more costly and that's another post entirely.
1) Get hold of a pair of Female-Male-Male "Y" connector (I'm assuning RCA connectors) and replace the jumper, leaving the females dangling. This will maintain your existing signal path and provide an avenue to send a full-range signal to another line-level device. Connect these to the subwoofer, adjust it's crossover (used as a low-pass filter now), level and other controls to blend the sub in with the system. That could be the most time-taking part. It was for me.
Total cost : A pair of "Y" connectors and a stereo interconnect, which you would have needed anyway.
That's all you can really do, The rest is for others or lurkers that might find themselves in a similar situation.
2) If your sub has line-level inputs AND outputs, you would could simply remove the pre/main link, put them in a safe place, and use two pairs of interconnects to send the signal both to and from the subwoofer. Depending on the sub, it may send back a fileterd signal (sans bass) back to the mains.
3) In the worst case scenario, there's always speaker level connections. It could be either connected in parallel with your main speakers (Subs have a very high impedance in their operating range so the load will be minimal) or, if they also have outputs, you can connect your mains to them so they get a filetred output (no bass), thereby relieving them of the onerous duty of having to reproduce the los lows. I've got a pair of Minimus 7's running off an old DBX passive sub and a Marantz 2270 that are quite happy with this arrangement.