the 3000DSP/3000D amp has 50 volts output max per channel.
the um15 adn um18 are both either 1 or 4 ohm loads. so they would have to run 1 driver per channel at 4 ohms.
um15 is blue line in 4 cubic feet sealed per cab. 50 volts. 2 cabs, 2 pi space.
um18 is red line in 4 cubic feet sealed per cab. 50 volts. 2 cabs, 2 pi space.
the green is about 4.5 cubic feet ported and tuned to around 25hz (with 20hz high pass filter set) with the saf184.03 driver mentioned before. it is 8 ohm so two could be wired in parallel and the amp run in bridged mode. the green line shows the maximum output while limiting VA from the amp to about 1200 or so watts. that's about all it can do. you could EQ the response to be anything that you want so long as you don't exceed the green line. the behringer has dynamic eq that could allow for some change in the frequency response as the level increases in order to maximize mid-bass headroom without clipping the amp on the low end.
Another possibility could be to build relatively slim (perhaps 12-16" deep), but large volume enclosures and place them in the back or back sides of the room. that would allow you to get a lot more low end with the um drivers for pretty much no additional cost.
the orange line in the image below is the same um15 driver, same 50 volts, etc. but in 10 cubic footer cabs tuned to 18hz.
then, as further migration upstream, a couple of sealed cabs with cheap PA460 drivers could be put in 3-4 cubic foot cabs in the built-in center and crossed in around 50-60hz. that would allow increasing your crossover frequency quite a bit and give a lot more punch to your system. that would require picking up another amp though. a 1000D should be sufficient, but i always recommend the 3000D as it isn't that much more money and allows for quite bit more options.