do you have a turntable near by connected to the mixer?
What mixer is it?
Any type of microphones hooked up to the unit?
The Guitar Mixer is already pre Amped, and so is your mixer so depending on what volume level you are with your mixer and Speaker together will control the feedback you are getting.
Note: it's never really wise to hook up Pre Amped Equipment ( or anything pre amped ) to Pre Amped Speakers. Unless you know how to control your volume, and stablize what your doing, and what your using it for.
Are you trying to hook up an EFX that your Pre Amped Guitar Speaker has, and your trying to hook that up to your Pre Amped Mixer, so that you can use it? If thats the case why don't you just get yourself a Mixer with Effects on it... Like the QFX 19???