Correct Way to Invert Phase

Reversing the positive and negative on the speaker will not harm anything. There is no way to reverse the positive and negative on the preamp cables. BTW, you might not hear any difference if you do invert the phase! Give it a try you have nothing to lose!

Yogiboy is correct. Rewiring the preamp-to-amp cables such that + is connected to - and vice versa would connect the signals on the preamp’s + outputs (the RCA center pins) to the amp’s ground, which of course would not work too well :-)

If the preamp-to-amp connections were balanced (which I presume they are not, since you are using a CJ preamp) interchanging the connections to XLR pins 2 and 3 at one end of the cables would do the trick.

Also, if by any chance your speakers are not passive (e.g., if they incorporate powered woofers or are otherwise AC powered, or if the amp outputs are connected to powered subs), I would check with the speaker or sub manufacturer to be sure it is safe to connect the amp’s + outputs to the speaker or sub’s - inputs, which conceivably could be connected to ground via some low impedance within the speaker or sub.

Regards,
-- Al