Pics of my gear, how would you hook it up?

Indeed left RCS goes to left MONO TRS and right RCA goes to right MONO TRS.
So one goes into channel 3 and one into 4.
The cable Coltrane09 linked is the one you need.

With (audio)XLR all you need to remember is “pin follows signal”. Meaning the pins (male connector) always “point” in the direction the signal goes. So output from your controller/mixer is always male, taking female XLR cable end. At the speaker end the inputs will always be female, taking male XLR connector. If you want to go from a microphone to a mixer, you start with a female XLR at the microphone and run a male connector into your mixer.

I think I explained this one in another post already. Assuming 1 sub and 2 tops:
1) run two xlr cables to the sub
2) plug the male XLR into the female connections marked “input”
3) plug the female XLR connectors from two cables into the male connections marked “output”
4) take the male end of both cables to one top end speaker each and plug the male connector into the LEFT hand female connector on the speaker marked “mic/line in”
Few things to watch for”
a) on the sub. Set the polarity switch to “out”, this should mean no polarity/phase switching is active
b) also on the sub. Set the High Pass Filter switch to HPF
c) on the speaker. Make sure the small switch next to the left input is set to “line”

Humming is usually a sign of 50 or 60Hz interference coming from the power end of things (ground loop). Often you’ll need to check your cabling and/or use what’s called an earth lift (which cancels the ground loop)

How loud is it? If it is only noticeable in a quiet room and standing pretty close to the speaker, it can also be a hum that is just in the speaker (or more likely the amp). However, since you say the hum just moves up the line if you take speakers out of the chain, I doubt that is the case here.

One thing to try is to hook up the controller and speakers to the same power outlet, sometimes if speaker and controller are on different “power-groups” it can cause some interference also.

I can’t imagine the 518s are not packing a punch. It’s an 18″ sub with 500Watt RMS. Not a lot by sub standards, but I can’t believe you don’t hear/feel anything. Especially if you are looking at 50-100 people. Obviously, if you are looking for “concert-level” sub-low, it’s not gonna happen with these speakers and even the QSC might disappoint you. But that is a combination of taste, your ears and your perception.

To get a 3dB increase in Sound Pressure Level (SPL) you need DOUBLE the amplifier power using the same speaker. Unfortunately, a 3dB increase is not perceived as twice as loud by the audience (our ears are just not rigged that way). To get twice the perceived loudness, you need about 10 dB increase in SPL. Meaning a bit more than 3dB + 3dB + 3dB or, in amp power, 2x + 2x + 2x. Total amp increase needed is 2x2x2 = a whopping 8 times the amp power to SOUND twice as loud.

Why am I telling you this? You’ll say the 518s has 500W and the QSC 1000W. So the QSC, based on amp power, can only be 3dB louder than the 518s. BUT, there is another thing to consider, and that is sensitivity (the efficiency of the speaker really). This is a measure of how much power is turned into volume.

And looking at the specs, the 518s runs at 129dB max and the QSC at 135dB. As I just showed you a 6dB increase would require FOUR times the amp power if both speakers were equally efficient. Since there is only twice the amp power, this means the QSC gets a lot more dBs per Watt amp power.

The answer to your question is the extra 500W worth it, would be a sincere yes (trust your ears though, not the figures!). You pay for twice the amp power, but get the loudness gain of four times the amp power.

One last thing on your current speakers, you might try setting the polarity switch the other way (don’t play music while you switch) and see if that makes a difference. I doubt it, but you never know.

Yep, could very well be. We did a studio test using budget, regular and high end cable and while I don’t have audiophile ears, it was absolutely audible. Now we are not talking about hooking up low signal micrphones, but high signal line equipment, but it still pays (also from a mechanical safety point of view) to buy at least a decent brand. There is no need for “monster”-like cabling, but cutting corners on cabling is not a good plan, imho.