With Main/Mon switch (Power amp Mode) depressed, one speaker connected to Main, second speaker connected to Mon, Main speaker has muffled sound.

It's always possible, but I doubt it's the fader. I'd suggest taking it to the shop for a hands on diagnosis.

If you are using the amp in stereo mode, then you will require a separate amp for the monitors and the input of that amp are taken using BALANCED TRS cables from the monitor jacks. If you are using the amp as MONO and monitor, then connect NO MORE than two 8 ohm speakers to the Speakon plug on the back labeled for use as monitor. Refer to your user manual... they have detailed photos of the connections. If you lost your manual, you can download another from the Behringer site. There are settings you need to do to configure the unit after connecting your speakers. The small slide switch has to be set to the center position for Main/Mon. Make sure it is centered as many people have had it slightlyu off and the system will NOT pass audio. You need to set each channel "monitor 1" level control to send the monitor signal to the MONITOR mixing bus. You will then use the MONITOR 1 slider to control the amount sent to the monitor. Be VERY careful of grounding the sleeve of 1/4 inch jacks that originate at the Speakon power amp outputs as the SLEEVE of channel A is the HOT side and you can blow the power amp for the channel... it is EXPENSIVE to repair.

I think that rather than the mixer, that your front speakers may have a limiter built into the high frequncey amp. You may be asking them to do more that they are capable of.

DO NOT parallel more than two 8 ohm speakers on either output and you will be OK. You could also use one speaker on each output. That would be easier on the equipment as it would be an 8 ohm load on each instead of a 4 ohm load on one of the internal amps. If you want to drive stage minitors, then you existing setup is fine and you will connect the monitors to the second speaker output and set the Mon switch as MAIN/MON.

Bridge mode you use chan B (left connector on the back) using a SPECIALLY wired Speakon. The impedance of a speaker connected as such CANNOT be LESS than 8 ohms.

In the Main/Mon mode, the two amplifiers are separated in function. The monitor knobs for EACH channel input control the mix sent to Channel A which is for the monitor speakers. The Mon1 controls the level sent to Output A for the monitor speakers. Mon2 fader is NOT used.

The individual channel faders control the main mix and the Main fader controls the level sent to Output B which goes to the main speakers.

The MON faders ONLY control the output to the MON L and MON R jacks, NOT the split use of MAIN/MON outputs. You MIGHT try varying the MONO fader and see if it varies the MON part of the split output.
Best to use the unit as MAIN L and R to main speakers and use a stereo amp or POWERED monitors driven from the MON L and R jacks. With this, the MON faders will control your monitors as you would like them to.
ALSO make sure ALL equipment connected together is powered from teh same recepacle power system. Powering interconnected equipment from receptacles at opposite ends of the room is asking for damage due to ground bounce and faults.
PLEASE read my tip on here about a HAZARD of speaker polarity if you use Speakon to 1/4 inch cables.

Please read a tip I just posted regarding a hazard of the Speakon to 1/4 inch cables when used with these PMP mixers.

If you are using your main speakers as stereo, then you will need either powered monitor speakers OR an additional seperate amp for them. The mon outputs on the mixer are LINE LEVEL and are not driven by a power amp. Use TRS (balanced) interconnect cables from these to POWERED monitor speakers.

You can parallel your two mains providing the parallel impedance is 4 ohms or greater (two 8 ohms in parallel at most). The the monitor speakers can be paralleled and the switch set to Main/Mon so that one side of the power amp drives the mains and the other drives the monitors.

To prevent damage to the mixer, make sure all speaker leads are secure before powering the system. Run the speaker leads through the handles of the speakers to prevent inadvertent unplugging due to personell tripping over cables.

DO NOT use the bridge mode in your use.

There are many things, including hardware failure to cause this.

Make sure that the PFL buttons are ALL up. The LED indicators on the right are shared with solo channel and main output.

REMEMBER that the MAIN 1/4 inch jacks are TRS balanced and plugging a mono cable into these OR st the other end plugging into a MONO will short out the balanced line drivers. Since this also goes to the power amp, that can ground out the signal to the amps. All the other 1/4 inch jacks are pseudo balanced and mono plugs will not cause a problem other than possible hum or noise levels.

Do you get anything out of the PHONES jack?

Note that the standby switch must be UP in the upper right corner... it kills the first 12 channels. ALso with two speakers make sure the MAIN slide switch is in the MAIN A MAIN B upper position.

Start with all your PAN controls at 12 o'clock.

I have repaired two of these units and a PMP3000. Great caution: NEVER pull or insert speaker cables while unit is on. Great damage can be done. IF the fuse blows do NOT replace it!!! Get serviced immediately as replacing the fuse will likely do more damage. The worst damaged one required replacing around a dozen transistors and other components.

The power amps in these have a habit of arcing out... NEVER store in a high humidity area!!!

Try a set of stereo headphones plugged into the L and R MAIN out 1/4 inch jack... If you have audio there your power amps have failed or the +/- 85 volt supply has smoked.

Make sure the three position slide switch is set correctly for the stereo usage, that is NOT the bridged OR the Mon mode.

MAKE SURE the pan pots are set to middle as they control the spit of the audio to the two channels.

This is a class "D" amplifier (switching) and there really isn't any fuse for a single channel.

MAKE SURE your speakon connections are good by swapping your speaker cables.

I repaired my unit and had to do that without schematics... Behringer will not give them out. Mine had a severe arc-over on one side power amp taking out about ten transistors and diodes and one swithcing regulator chip. Had to cut out the "cancer" of the burned board.

This unit uses a complementary output stage that is made up of one transistor on one side and a pair in parallel on the other. This drives the inductor/capacitor filter... I think the switching frequency is between 100KHz and 1 MHz as I recall. All those transistors were gonzo. It also fried the main power switching transistors in the power supply, several smaller transistors, diodes, and the switching regulator chip.
Also two 10 ohm power resistors were fried. They soft start this unit by charging the main caps via two resistors that limit the inrush current.
A small switching supply has to come up first before the main supply which sends the DC to the amp section.

Using the mon out and the proper balanced cable follow the instrcutions found in the manual section 61.

Here is a link to the manual.
http://www.behringer.com/EN/downloads/pdf/PMP1000_P0566_M_EN.pdf