I have Altec 604E from 2005.
I never listened to Altec GPA. I don’t know how do they sound. It can be different from old 604s.
My 604 is very easy to drive and sound is very dynamic without feeling of any compression. The bass is fast and has texture but not deep. In my 9 cubic feet cabinet it doesn’t go deeper than 40hz. High frequencies go up to 10khz, so I use supertweeters.But new GPA should be more extended on high frequencies.
These speakers wonderful for jazz and vocals. They can play rock and classical music too.
They can play in midsize rooms. Even 200 square feet is ok. Because the are coaxial they can play from 8 feet distance.