You could switch to a more laid back amp that may help subdue some of the brightness. You could also send them to a professional DIY'er who could change some aspects of the crossover, like reducing a resistors size, which would help.

I am a little surprised at your problem. I had never heard a bad thing about Platinum Audio. I believe the creator of Platinum Audio moved on to Soliloquy so maybe you could even contact him and see if he has any recommendations. I'll look for his name and post back if I find it.