You could try toeing one set of speakers in and the other out, both slightly to possibly alleviate some of the afects of the comb filtering. The biggest thing with comb filtering is that is more of a measured concern than an audible one, at least according to Alan(resident expert) and my experience with the VP150 which is said to be very bad for comb filtering yet sounds good to me.
The biggest thing is do the stacked speakers sound any better than they do when they are run as singles or side by side, if they do then you have an issue, if they sound better or just as good than there is no reason for concern. It is all about you and how pleased you are with the sound, nothing else really matters.
