In a way this is sort of a problem, sort of not..

I have a pair of gemini active speakers...

There is a line level and and a line out level..

For some reason I need to have these both up to get some sound out of speakers?

I mean nothing is plugged into the line out level but it still needs to be up

Also, I notice a reasonably loud electrical hum with no music playing and the levels set past half..They seem okay at half?

I have checked the michrophone cable and the power lead but to no avail.

Regards
Andrew

Are you running long cables to the speakers? Are they on TRS jacks or XLR? Are the lines balanced? If not it could be noise from the cables. Do you have balanced outs on your mixer? If not a stereo active DI box might help by running short outputs from your mixer into that and then your normal (balanced) runs to the speakers.

If it's the GX series then have a look here

http://www.geminidj.com/product_manuals/gx_man.pdf

The input level knob controls both the input and the output level of the line. The knob to the right of the line out ports is actually the speaker level (amp gain) This allows you to get your gain structure correct by feeding a hot enough signal to the speaker and then you can set the level of the output on the speaker output knob.

Setting this up correctly may well reduce the noise you're hearing too.

I use balanced out on my mixer

Both neutrik xlr to xlr cable - High Quality.

I can't see how this could occur?

Then it's not your cables if they're balanced.

Have you tried with a different mixer to eliminate that element of your signal chain?

Is that the correct manual for your speakers? Does it explain which knob is which? That would seem to be why you need to have both knobs turned up.

You need to try turning up your mixer so it's giving just under 0db, then turn up the line input level until it clips the light on the speaker, then back it off a bit. Then turn up the speaker output level until you're comfortable with the volume. This will give you the highest signal to noise ratio and should minimize the hum. By keeping the input level low and turning the speaker output up high, you'll just be amplifying the background noise.

Could also be a ground hum. Do you have the facility to lift the ground on a switch on the speaker?

The hum is usually an earth fault within the PCB that controls the input and output gains within the speaker unit, always make sure your cables are less than 10m for the speakers, a common problem with earth hum in the phono cables that go from your cd-player to mixer, mixer to amp etc..., it might be an idea to check all of these before going any further, a phono cable hum dosnt have to be a direct cable it can be any phono cable that is on the mixer, cd-player, x-over, amp, GEQ etc... worth a shot!