Altec Model 19 Soundsystem with Subwoofer pairing

Hi everyone,

Long time lurker of this site and finally now posting! Over the last few years, I have become obsessed with what high-quality speakers can do, and during that time, I have found (4) Altec Model 19s to emulate a modern club-level sound system with the aesthetics of vintage. We raised the Altecs so that the HF Horn would stand about 8 feet high and power all speakers with an SS Yamaha M4 amp (we are going to upgrade this when money permits). The wood floors are roughly 30x30 feet, and our capacity is around 200-250 at peak hour. During the first event, we blew a woofer on one of them from cranking the system too hard, so we need to dial that back, thus adding additional low-end to the mix is necessary to take load off the 19s. The hope is that by adding two subs on either side of the room (with each 19 positioned in a corner of the square) we can get below 30 HZ but crossover (via a DSP) at a frequency less detrimental to the long-term health of the 19s. 
 

My question: Does anyone have a good subwoofer rec that might pair nicely with the 19s? Open to modern, was thinking of something the JBL 4645 ilk (theater quality), but very much green here. If there is a DSP recommendation as well, that would be appreciated as a place to start looking into. Thanks in advance for the help!