Roland Cube 40XL connection to mixer

That sounds like a compnent failed in the preamp section. This will not be user servicable and will need to go into the repair shop.

Yes, there are adapters available.

Gain too high with single coil pick ups?

Did you check the solder connection on the jack where it connects to the motherboard? That's a high stress point and could simply be a broken contact that needs to be resoldered. Should be 3 or 4 pins likely.

have a look here:
http://www.ehow.com/how_8698510_fix-roland-cube.html

although it doesn't mention the fuse you will probably find it. Sometimes there is a fuse access on the outer case.

You are connecting a NOISY unbalnced line to a balanced input. Buy a DI box and then use an XLR to XLR to the mixer inout and run the MAC into the DI with "ground lifted". You will need to make a resistive mixer between the headphone jack and the DI as well. The headphone jack is STEREO. You COULD try a STEREO 1/8 inch splitter to 1/4 inch left and right cable... you MIGHT get away with that. Right now the left and right amps in the MAC are fighting eachother.

You would NOT connect an audio signal to the "send" jacks... those are OUTPUTS... In doing so you are driving signal backward enough to get to the headphones. Sends and returns are NORMALLY used on a mixer for external effects. You should either be using a LINE input jack or one for "tape input".

Heree is link to cube 40... not sure what the XL suffix gets you...

http://elektrotanya.com/roland_cube-40.pdf/download.html

Scroll down to the little link "Get Manual" and click on that.

Yep! Get a decent audio interface that attaches to the PC with a USB connection and uses BALANCED lines to your mixer. I use a UA-25EX Roland/cakewalk as an example. (about $200 and there are cheaper units such as M-Audio Fast Track MKII ) Note these interfaces BOTH record and playback with two channels for stereo.
Essentially the audio board that is in PC's and Laptops are too near the switching noise of the processor and some of that is just bound to get into the audio lines as they don't even have balanced audio lines from the computers. Make sure you use either TRS or XLR balanced cables from whatever interface you get. Also connect ALL interconnected equipment to the same power source or receptacle. You can try to reduce the buzz with the mixer EQ's with what you have, but that sacrifices audio quality.

Assuming you plugged into the headphone jack, the switch contacts on the jack have apparently failed.

A cheating way to disable the speaker is to plug in a plug that has no connection to the terminals into the external speaker jack.