Feedback from bass amp

Probably the soundboard is resonating. If you out your hand on it or the strings it will probably stop resonating if it was just on the verge of feeding back. This is the main reason why solid body guitars were invented in the first place.

There are ways to minimise this, however. Speaker placement, eg have the speakers beside you and facing away to the audience. Stand further from the speakers. Use an eq pedal and lower the gain around the resoant frequency. Use a chorus/flanger pedal. This can help smear the frequencies and reduce feedback. If you google "reduce acoustic guitar feedback" you should find other ways. Also, if there is a way to increase the gain of the pickup by placement, do that.

Proper level setting of the mixer is important. HOWEVER if the clipping is occuring at the speakers the only possible problem MIGHT be that you have a supersonic feedback that is saturating the speakers above your hearing range, Be sure you don't boost the highs too much with the EQ as this can cause the feedback that you can't hear. Also make sure that you power the speakers from the SAME receptacle as the mixer, even if it means running an extension cord to bring power to the mixer. This is to avoid a low frequency hum and common mode distortion/damage. A low frequency hum could cause the clipping.
It would be a good idea to get a sound meter to check the sound level. You should be able to reach 85 Db from this system without clipping. If you need more than that, you MAY need more speakers if the band instruments are too loud. Also if the band has amps that get into the vocal microphones that adds to the clipping level... make sure the mics don't "hear" the band instruments. Make sure your speakers are toward the audience from the mics to avoid the supersonic feedback problem. If the vocalists can't hear themselves with that configuration you need to set up seperate stage monitors.

There are some pretty good videos on YouTube about proper mixer level setup.

If the guitar can "hear" the sound, the strings can resonate from the sound and result in feedback. Running at maximum gain is asking for trouble. That causes distortion which drive the speaker as a square wave and worsens the problem. Also back off the treble as you can have supersonic feedback that you can't hear but will heat up your amp.
Test by allowing the feedback and then dampen the strings... if this stops the feedback you need to move the amp away from the guitar. Being a passive pickup makes NO DIFFERENCE about the problem.

First of all I suggest that you try your guitar through another amp you know works without the feedback. There is a very slim chance there is something wrong with your guitar but I would say no. You didn't mention what kind of amp you have: solid state, tube or hybrid. It would help if you tell us what brand and model number your amp is. Try setting your amps controls the way you usually have them and using a long cord that works properly, isolate your guitar by putting it in a room with the door closed by running the cord under the door. Is the feedback still there? Try playing with your back to the amplifier. Is the feedback still there? You need to identify the problem. Is it your guitar or is it the amp? Here is a link to tube amp troubleshooting:http://www.geofex.com/ampdbug/ampdebug.htm Here is a link to guitar feedback causes:http://www.guitarsite.com/discussion/messages/23765.shtml If you decide it is your guitar you can have a look for broken solder joints etc. There are wiring diagrams on the internet. If it is your amplifier, you should take it in to be serviced. Take it to a dealer that carries and services that brand. Unless you have all the tools, a schematic and electronic troubleshooting experience you are in danger of injuring or killing yourself or making the problem worse. Hope this helps.

I dont know this model of feedback destroyer, but i'd like to tell you this. I'm a service technicien for a touring company. We found that in the last 10 years Behringer quality drop down so much that was making alot of their components useless. To cure such problem usually found with monitor, i'll recommand buying a 31 band EQ and adjust the mid high freq level section. You could install it with inserts cable just on the main vocal entry i guess, also you should give a try to this way of connections for you feedback destroyer. It might be the way you're hooking it. Send a draw of the actual connections...

This is clearly a microphone problem. You need to review your mics and use a suitable type. Ensure the speakers are not aimed at the mics too.

it could be your guitar pickups. try using another guitar or getting your pickups waxed at your local music store. also if your playing in a small room kind of loud, or if your using single coil pickups (like fender statocaster pickups) you will definitely get a feedback noise. if that does not work let me know.

Start with a clean setting - zero cut and zero gain on the Eq. Play a song close to the amp, and then move away, playing the same notes. Do you still get the distortion? Change lower the input gain and repeat the test. Lower the output gain and repeat. Are any outboard distortion pedals in the mix, such as overdrive? Eliminate them. Run straight through from your axe to the amp. Check for any distortion settings. Set everything including reverb to zero. When the distortion disappears, this is your base setting.
Change only ONE parameter at a time. When the distortion comes back, you have found your problem.+

May i suggest removing the equilizer unless you can adjust the settings for each amp seperatly. The amps themselves will allow you to add bass boost and treble boost. I think the problem your having is trying to run to much bass through your system causing it to distort badly.