Congrats on placing the order, I keep thinking about your dilemma and I believe the benefit of keeping the 600 at least for now is that you will be able to hear how the 2 subs sound together in your room, if the combination is that great your wife might be convinced and might relent. If not, you could always put the 600 up for sale later after auditioning the combined impact of the two?

"If you try to turn toward it, you go against it."