I've got my 15A external power supply now and it's working with a 16Mhz crystal ok, I'm getting avg. 360MHash/s with this installed, running at 1.35V - is this a normal speed for a 16Mhz crystal?  Little or no hardware errors with this config.

If I put a 20Mhz or 24Mhz crystal in, the light doesn't go off and the BE isn't recognised by the BFGminer software.  Does that mean it's the wrong type of crystal or any suggestions as to how I could make the higher speeds work, even for a short time?