Graeme;
This probably isn't it, but I thought I would mention it just in case. On some boards there is a low frequency filter switch on each channel that rolls off starting at around 50Hz (especially useful on vocal mic channels). If the person running your PA has that switch turned on for the bass guitar channel, that might result in bass-light output. Just speculating.
Also, if you are micing the bass cabs, make sure you're using a mic with a flat low frequency response curve. Lots of mics roll off on the lower end of the band.