I have run some compression drivers at much below their rated frequency, but at a fraction of their power rating.

Let me put it this way, a 288 is approx 15 watt rated, at 250 hz I would guess your max might be between 3 and 5 watts before ending the life of the diaphram.

If you need to use a horn lower than the compression drivers you have available, why not use a cone driver in a horn!!!!

Just my 2 cents worth, how lucky do you feel?

I pay good money for my Great Plains 399 drivers(288-8K clones), I can't afford to replace diaphrams just for a experiment.

Sorry to be a spoil-sport!

Ron