For my P2K GP30, I initially had a cut down iPhone 4 speaker installed under the rear fans, but I've recently switched it to a single Sugar Cube under the exhaust stack as that's where the sound should be coming from. Personally, I'm not sure which sounds better, but I like the sound better coming from the correct area.
I've used the iPhone speaker for all of my installs, save for a P2K S3 and my fleet of Bowser SD40-2s which required Sugar Cubes. Surprisingly the SD40-2s are ridiculously tight for space!