Rewirin a Sub

I need to rewire an 8-ohm 15-in sub in an existing cabinet. It currently has a single 1/4-inch input connection and I want to add a second speakon connection as well as a speakon output that will allow me to wire it in series with another 8-ohm cabinet. In any case, I've got a bunch of 12-ga copper wire (solid core) around that I was hoping to use (basically left over Romex). I saw in another post that stranded oxygen free wire was recommended; but just curious if the standard solid core stuff would be alright for this purpose--basically between the connectors and the speaker terminals. Secondly, when I'm making the connections, can I use wire nuts for the cable connections or should these all be soldered? Thanks Alex