I've also got a GD87 and, generally speaking, the volume is just too soft.
It's not only the polyphonic ringtones, but for me it's also not possible to use it in a somewhat loud environment like a pub. I never had problems with my phones before but am kind of disappointed here.
The standard ringtones are rather loud, though - so my solution is to switch do ringtone #1 in louder environments and switch back to polyphonic tones later.
Tom