Plenty easy to design a reflex box tuned to 55Hz. I would think for an acoustic you would want also to use something for the treble. With electric guitar amps setups they generally don't use tweeters or compression drivers because the heavy distortion would take them out fast, but with acoustic guitar, you want to hear that clean tone, so I would either use a coaxial speaker or two way with a comp driver and wide dispersion horn. To design the box, I would recommend Win Isd or something along those lines. Here is a simple calculator for Reflex boxes which is web based. I have used it with decent results. www.mh-audio.nl/ReflexBoxCalculator.asp
Lots of other useful calculators on there too.