Graham, I will try and explain what I think works best...Over gain increases noise..

Try to maintain "unity" (0db), usually 12 noon on the volume knob..

On mic channels..set the volume at unity, and adjust the gain control to acheive the proper volume input...

On the back end...output volume, do not go into the red..stay well below red..in the green prefered..If you need to push the output of your mixer beyond the amber caution before clip...usually means your PA or powered amp/speaker is not powerful enough for your needs..

Sure you can push a liitle more on the front end (gain), but it will induce noise in the noise to signal ratio...

Another common no no, folks will over use EQ..sometimes EQ on top of EQ..This will overdrive a signal to unuseable limits..Learn to adjust EQ, is one of the best things you can do for desireable results''

As an example..a standard 3 band channel EQ, can be used wiser on most mixers,by cutting eq sections rather than increasing the frequencies of a section...For example cutting mids can be more effective than increasing EQ's on the low and high frequencies..By cutting the mids and raising the gain slightly you will get better results, without overdriving the signal..

Effect send levels will also distort and overdrive the signals..be careful with your levels..

A digital mixer will have more negative effects with all of the above than a analog mixer...It is more noticable with digital, but be assured the noise level is there on the analog too..