It seems like the main outs would give the full main mix, or you could could you use the efx send jack. The former may require a re-rout back to the main amp ins. The latter would require use of the efx send knob on the individual channels that you want assigned to the subwoofer. Unfortunately I think this method would eliminate your capability to use the internal effect, although I'm not entirely clear on that from the manual.
I don't see why the monitor send and out couldn't be used for the same purpose, without interrupting the effects use. Unless, of course, you're already using that send for monitors.
Either the efx out or monitor out scenarios are similar to the aux-fed subwoofer method that many pro audio guys use and is an advantage because it allows you to pick and choose which inputs to send to the subs. Typically only those low frequency information - bass, kick, maybe some toms, keys, maybe guitar.
Big BUT here...regardless of which method you use (main outs, efx out, mon out) you really need to use a crossover to split the main out signal - highs to your mains, lows to the subs. The image you posted shows a crossover in the signal chain with the high frequencies being returned to the amp ins. If you do not crossover the main out signal then both your tops and subs will be reproducing the same signals. This is generally not a good thing because some frequencies will cancel due to time alignment and/or phase issues, and even if some frequencies combine additively it will likely not sound good.
If you don't have crossover capabilities then I wouldn't recommend trying to run a subwoofer. Even with a crossover, I don't know if your extra cabinet will do the job you're probably hoping it will do. Pro audio subs (even inexpensive, not-so-pro subs) are efficient and high powered and will handle the low end of a complete band better than a instrument cabinet tuned specifically for bass guitar.
Title says it all. I have the mixer, but can't see where you'd plug in a powered subwoofer. The manual has a diagram that implies you can do this via the effects jacks, but it's unclear what to put in between the mixer and sub.
I have a non-powered LDS 15/6 bass cabinet that's not always in use. Could that be used as a PA subwoofer? I'm currently using a Genz Benz ShuttleMAX12 as a preamp, and a QSC PLX3602 as the power amp. Only one side is used for a bass cab. Maybe run a line from the Mackie effects send jack into the QSC and run the 15/6 as a sub in that configuration?
