Altec made some compression drivers that were made for "full-range" public address systems. The drivers have phenolic diaphrams and are made more durable to handle the lower frequencies. These are not considered "audio-grade" but were designed to be used in areas where speech reproduction was required to cover a large area.I'm not trying to rain on anyones parade, just saying that using a Altec 288 below 500hz was not what they were designed for.
Also the many lower frequency horns (1003B/8003B/etc) were actually used at 500hz crossover unless a different driver than the 288 were used.
Ron