Re: subs for small venue
Doug,
Usually the description "boomy" indicates a problem where the subs and mains overlap, and the output of the ported tops usually don't align properly with subs without some time delay and frequency adjustment to get rid of the crossover peak "boom".
It is common to have to pull -6 dB at a fixed crossover point or underlap, for example cross subs at 80 Hz, and tops at 110 Hz to eliminate the boom.
The output of subs is determined by displacement (cone movement X diameter), given the same excursion, 2x12" is a bit more than one 18", 2x15" a fair bit more. Smaller drivers may go lower for a given box size.
That said, to reproduce low bass at the same level as mid bass usually takes more cone area, a pair of 18" may be "about right" for a pair of 15".
Your new tops may have a fair bit more output than the old tops, and will need more bottom to "keep up".
If the powered subs don't have built in DSP (digital signal processing) designed specifically for your top cabinets, you will still need a crossover, EQ (and possibly delay) to avoid a "boomy" overlap area.
Regardless of the sub choice, for small systems I generally prefer using a pair for the convenience of using them as pole mounts, eliminating the trip hazard of tripod stands, and providing a consistent set up, rather than trying to figure the best compromise placement for a single sub at each venue.
Art
