Well, an update - got chance to run the vocals through a few FX pedals last night, including preamp (with tube saturation emulation). Sadly, nothing was working.
The results were thus:
channel 1:
Shure SM58 Beta > XLR-to-XLR > PA mixer = superbly clear, very nice 'normal' live vocal sound.
Channel 2: SM58 (same mic) > XLR-to-TS jack > FX stompbox (overdrive pedal) > TS jack-to-TS jack > PA mixer = very poor sound quality, much weaker (lower vol) signal going into mixer, tone coloured (very muddy), VERY LOUD FEEDBACK at anything other than the lowest drive / gain settings (which meant we couldn't turn the mic up much louder than normal (non-mic'd) spoken word level)
Channel 3: SM58 > XLR-to-XLR > Behringer UltraVoice Digital mic preamp > XLR-to-XLR > PA mixer = amazing clean sound, better BY FAR than the original direct-to-mixer control measure (channel 1), cleaner, clearer, huge signal boost (the Behri UVD is no longer made, but it's an amazing product if you can find one on eBay or somewhere) (superb for recording), but sadly the tube emulation did not distort the vocals enough, only very very mildly, which sounds great recorded, but not for use as a 'live' overdriven vocal effect.
So... back to the drawing board... any thoughts / ideas anyone?