750-850Hz is what I would aim for at first. We shall see!

The tweeters can roll in about 3.5KHz and make up for the drooping 288. But they won't extend the FR any higher. 15K will be it. Or maybe bring them in much higher with a HP shelf on the 288. TBD.

Getting on out past 20K with good efficiency is not easy. The T-350 may not be the right driver for the job.