The Fostex FE87E has a small cone and modest x-max. On an open baffle with no protective highpass filter, the math says it starts to go non-linear at about 7/10ths of a watt at 200 Hz, and at about 2/10ths of a watt at 100 Hz and below. In other words, it will theoretically start to lose clarity at fairly modest sound pressure levels when used as you describe, though it may be designed to "gracefully" go non-linear and thereby give you a few more useable decibels.
Duke