Titanium high frequency drivers not working. Is there a fuse in crossover?

These are passive speakers, so if there is a fuse at all it will be as a high frequency protection circuit on the crossover.

diaphram or crossover..

The capacitor determines the cutoff frequency of the filter used to either block or pass frequencies above or below the intended design point. Generally, electrolytic capacitors are used for this, due to their capacity for a given size. electrolytic capacitors are manufactured with two foil plates separated by a gauze soaked in electrolyte compound. This electrolyte compound forms a one molecule thick insulation layer. If this electrolyte dries up, the cap will tend to fail, either short or open, usually short. Because the insulation so thn, it is very much affected by the applied voltage (usually DC). Most electrolytic capacitors are used to block DC. these capacitors are usually pretty sensitive to the applied voltage. Most electrolytic capacitors have a WVDC rating, which stands for Woirking Voltage Direct Current. The higher the WVDC rating, the largewr the capacitor for a given capacitance. Also, these capacitors have a very wide tolerance because of this. Typically, this tolerance is +10%/-80%. This rating would indicate that at close to the WVDC rating, the capacitor will be within 10% of its rating. If the capacitor is used in a circuit where the WVDC rating is much higher than the circuit voltage, their will not be enough bias voltage to form the one molecule thick dielectric (insulation) needed for the rated capacitance. So, if the capacitor is rated for 100WVDC and is used in a circuit that only has 40 volts, the effective capacitance will be much lower, and therefor, the crossover frequency would be affected. If the crossover filter is a high pass, then you would risk damage by allowing too much low frequency content to get to the mid/high frequency driver(s). For a low pass configuration, too much high frequency material would get to the woofer, which might affect imaging, but should not damage the driver. Also, electrolytic capacitors come in many varieties. Almost all of them are polarized, meaning that the terminals are marked plus and minus. Capacitors for crossovers are not typically polarized. Here is a link to some non polarized electrolytic caps:
http://www.parts-express.com/term/crossover-caps?c=293,1385

Most audio enthusiasts prefer the sound of polypropylene, polycarbonate or polystyrene capacitors over aluminum electrolytic caps, which also have the advantage of not drying out over time, so they last longer. Also, aluminum electrolytics tend to fail if they do not have a bias voltage applied for a long time. Here is a link to some film cpas for crossovers:
http://www.parts-express.com/term/crossover-caps?c=294,1382

Please check the wattage of the replaced speakers, now check the cross over in the speakers, if the capacitors are short the voltage might flow into the tweeter coils to burnt them out. Also you can try and reduce the high frequency and also see if the tweeters heat up . If so use high wattage super cooled tweeters.
Check finally for fault in the amplifier output- high voltage/current.

Testing the driver with an ohmmeter is incomplete test. Sometimes the magnets break off the drivers rendering no sound. Sometimes the voice coil shorts and if you don't interpret the ohmmeter readings correctly you can overlook a faulty driver. Remove the horn and look for damage such as lost magnet from the back. Also the peavey cabs have a protection circuit so if you did not try another speaker to the same wores going to the driver, it may be the Polyfuse on the protection circuit or other components.

There is no crossover arrangement. On the PMP5000 they have a tunable filter INTENDED to do low pass for a subwoofer. When selecting this mode for a subwoofer The frequency of the low pass filter can be set between 30 and 200 Hz. This feature is NOT available on the PMP3000.

There is nothing related to a crossover in these amps other than what is mentioned above.

The MONO out is merely an analog summation of both the L and R channels.

The best way is to use a two-way crossover between the mixer and the amplifiers. In your case, the low output from teh crossover would go to the powered subs, the highs to the QSC. Use the crossover adjustment to set the frequency crossover point to get the sound quality you require.

Dan

Mackie 1604 ---> Crossover --- Highs ---> QSC --->Speakers
'
-----> Crossover --- Lows ---> Powered Subs.

You would send high as well as low to the sub and unless the sub had a built in crossover it COULD get muddy sounding. The specs SEEM to indicate the 118 DOES have a low frequency crossover so it SHOULD be OK... How would it sound? An 18 inch sub normally adds to the sound of a system... You MAY need to PAN over to send more power to the one side with the extra load on one side. Often people use two sub's... one for each side so it is balanced.

You should order an exact replacement from Mackie since this is a reference monitor.

Consider putting a limiter or fuse in series to protect the speakers if this was due to overdriving it.

The chokes blocks high frequency, the capacitors block low frequency. You would need a digital multimeter to check for continuity. Make sure none of the resistor is burnt, which may not be obvious.