One of those installation related details but your saying that you can't get exact dimensions concerns me as if it is not easy to pull out the existing subwoofers to measure the enclosures then that could be very relevant. I've seen subs under stages that were apparently put there when the stage was built meaning they fit but without tearing up the stage you'd have to break them up in order to get them out and would then be limited to putting smaller boxes back in their place. What matters is not the size of the existing subs but what dimensions you actually have to work within for anything that might replace them including the physical installation.
Do you have any feel for the improvements desired? Are you looking for some specific output level or if looking relative to what you currently have, a bit more output, twice as loud or even more? Do you have any specific goals in terms of the low end response are you looking to obtain?
Keep in mind that higher output and lower response from subwoofers under the stage is also going to mean more LF on stage. Might there be any need to consider some form of directional subwoofer array? What related signal processing is available?
