trouserpressface wrote:So at one point we had 8 acoustic guitars, plus bass and elec all going into the same desk, with whatever guitar leads I could find stretched across the stage like spaghetti.
As others have said, in the ideal world you should get a source list in advance of the gig so you can plan your equipment requirements and stage box plugging etc, and won't get caught out when 8 guitarists turn up at the same time looking for somewhere to plug in!
'Spaghetti' across the stage is never a good idea -- you really don't want people tripping over cables. This is partly because it's a great way to break the cables, but it could also break any connected instruments, or worse, break someone's neck -- and you really don't want to have to fill in all those public liability insurance forms! That is a very painful road to go down...
But absolutely do make sure you have appropriate PL insurance in place because accidents can and will happen regardless of how careful you are, and when they do they will come looking for big compensation payouts.... Being a member of some organisations (like the MU) can provide free PL cover, or you can get it via specialist brokers.
I would usually recommend that you run your cable routes along the front or back of the stage area where possible, and certainly never across the main paths on/off the stage. Run the cables in straight lines and tape them down where practical, too.
Often it can be advantageous to run one or more 8-way multicore tails from the main stagebox across to the other side or rear of the stage, just to reduce the number and length of individual mic cables running across the stage. Coil any spare cable neatly at the source end, not the stagebox end, so you can move source positions easily if necessary.
Where instruments need to go straight into DI boxes, place those by the musicians' chairs, music stands, etc, as has already been said. If they need to go near the amps, place them adjacent, but beware stray magnetic fields from the amp main transformer inducing hum into the DI box's audio transformer.
Individual DI boxes are usually more flexible, but if you have a lot of sources in one area (eg a Wakeman-style keyboard rig) it can be worth hiring in a multichannel DI box. If the DI box has a ground lift function, start with the ground connected and only engage the ground lift if you find a ground-loop hum problem. With guitars plugged straight into the DI and no onward route to an amp, it is very unlikely that you'll need the ground lift at all.
If an instrument or source is already actively amplified (eg, active guitars, guitars with built-in preamps, keyboards, drum machines, DJ mixers etc etc) then a passive DI box is fine as the source won't be worried about the load impedance. For normal (passive) electric guitars with magnetic pickups, active DI boxes are generally better as they usually have a higher input impedance (1M Ohm or so) which will maintain the guitar's tone better... but they will need powering via battery or phantom power. Piezo pickups typically need a very high input impedance (10M Ohms or more) which us why most are used with dedicated preamps.
The sound was ok, but very muddy, so after some reading this morning I think some of the guitars were impedance mis-matched, as they were piezo pickups going directly into the desk. Is this right?
As someone else has already said, the mud was more likely just because of the number of guitars and unskilled players. With acoustic guitars it's usually necessary to roll off a considerable amount of low end to get that mud under control!
However, as mentioned above, piezo pickups often need a very high input impedance to give the best tone, and neither the mic nor line inputs of a normal desk will get anywhere close. However, if the guitars had onboard preamps you'd have been fine.
H
beginner guitarists all playing much the same thing at the same time! So for next time, here's what I would do - assuming you don't have time to individually set up each group with a leisurely sound check and it's all going to be a bit rushed 
