My first high-efficiency speaker was a GPA 604-8H II with the standard GPA crossover. I built a JE Labs style open baffle and drove them with 300B SE amps. Unfortunately they never pushed the right buttons for me. I had plans all set for building a set of large MLTL cabinets but ended up selling the drivers instead. I do understand the appeal of the 604, and I strongly suspect a vintage 604 would sound better than the GPA version. However, I could never get over what I heard as some sort of roughness and a dip in the response in the crossover region. Also, despite the "point source" advantage of the 604 coaxial design, it was not as coherent as my Jensen P15LL/802/32 speakers.