You can invert phase in any cable in the chain to get the result you want, but that's not the best way to do this!
As has been noted in the thread, if you wire a special cable you'll need to be sure to mark it and make sure you never lose it or you'll be back at the soldering iron putting things right.
The much better solution is to open up the speaker cabinet and reverse polarity internally at whatever point is convenient, usually by swapping hot and ground at the input jack. Now, all the cables (preamp and speaker) are stone stock and you can sub with something from the big cable bag we all have if your regular cable has a problem. The change you made is safe and secure inside the cabinet where nobody can accidentally knock something loose or get it hooked up wrong.
The only way that this would cause a problem is if you share the reversed cabinet between two multi-cabinet rigs. That's pretty unlikely for most of us regular mortals.
As a side point, if you were going to hack a cable, it's much easier to change a speaker cable than a preamp or instrument cable. In the speaker cable, both conductors are equal sized regular wires, so no special work is needed to make everything fit. In most unbalanced shielded cables, one of the conductors is a regular wire and the other is the braided signal shield. If that's the case, you'll probably find it's pretty hard to group and attach the shield conductor to the center pot of the plug without inviting shorts against the plug body. You probably won't notice a shielding problem with a line level cable running between preamp and power amp, but if this were an instrument cable, you would have just defeated the shielding with a reversal like this.
David Fung
