Wow. Just got my PF-500 and I'm very, very impressed. Great tone, great EQ shaping, and great compressor.
Here's a clip I recorded using my P bass with flats. I play a riff first through my FMR "Really Nice Preamp" which is known for its clean flat tone. Then I play it again through the Ampeg (I used the DI) with the EQ set flat. If you listen with good headphones, you can hear the Ampeg has a nice warm tone with a slight boost to the bottom end, even when set flat:
http://atkinsonbasses.com/storage/audio/pf500.mp3
I'm very happy so far. Tomorrow I'll try it with a cab...