Hmm this is new to me... I ALWAYS try to get my input as high as possible... how can this be true... Im sure that its none of the gear, and just a question of getting very precise levels...
Well, maybe I could have expressed it a little better ... ;-)
Think of it this way: All electrical curcuits pick up noise. Mostly from long runs of wire that act like antennas. So you want as much signal as possible going through the wires relative to the noise. This is called 'signal to noise ratio' (duh!) So always turn the output up as much as possible before turning up the input it's connected to. If your input VU still isn't going near the red, *then* begin turning up the input gain until the VU start peaking.
Now, A guitar.. thats instrument level right?
Right. You are basically dealing with 2 types of signal: guitars and everything else. A normal electric guitar has passive electronics (no batteries) so it needs to be plugged directly into a high-impedance (hi-Z) input to 'load' the guitar's curcuit in the correct manner. Otherwise it will sound dull and lifeless. Typically your guitar will be plugged into a 9V-powered stompbox or amplifier input. Once your guitar signal is past this first hi-Z hurdle it can be considered a line-level signal just like everything else.
Is +4 line an - 11 instrument?
These are both line-level signals. +4 dB is considered the 'professional' standard, whereas -10 dB is the 'consumer' standard. If you're mixing pro-grade and consumer gear then switch the pro gear to -10 dB if possible. That way you can keep your signals as 'hot' as possible relative to the 'standard'. If the gear doesn't have -10/+4 switch then don't worry about it. Just keep things as hot as possible and everything will be fine.
Just one thing to look out for: If you plug a pro +4 dB output into a consumer -10 dB input, you *can* risk damaging the input if you turn the output up too loud. So, in such a situation, start with both input and output at minimum gain, then slowly bring up the output until you get a good signal on the input. The pro gear's output will probably only need to be turned up to about 1/3 of its travel.
Consumer gear going into pro gear is not dangerous for your gear, but you may need to turn up the input gain quite a bit to even register a signal, even with the consumer output maxed out.
However, I suspect that actually everything is instrument level...
Nope. Everything except passive electric guitars (and certain types of microphone) is line-level.
Both my EDP effects (the redsound federation and the Roland ef 303) are actually aimed at DJ's... therfore have phone plugs... so what does that make them?
Probably consumer line-level (-10dB).
OH.. And one more question about SEND LEVELS:
One each channel of the mixer there is a send knob, but there is an overall send level.
What is better, keeping the overall knob really high and just dialling in a tiny amount of level per channel?
OR the reverse, turning the send FULL UP to send to the loopers, and having the overall level really low...
OR... Having both low and turning UP the EDP input level control,
OR having both really HIGH, but keeping the edp level really really low...
A send is just another kind of output. So hot as possible (within reason). I'd turn the channel sends up to about half or 2/3 full, then the master send up until the EDP registers a good signal without turning up its input gain much. That way you have some 'tweak room' at every stage in the chain.
Hope this helps a bit.
Ian