You said you are disappointed enough already, but unfortunately I’ll have to add to that disappointment.
What you are trying to achieve is not gonna work. And it’s not even gonna work with subs with built-in cross-overs either.
There is no EQ on aux output on most mixers. Only channel EQ (and the odd mixer – usually the powered ones – which have a small graphic EQ on the master out).
While I can see somebody THINKING they need separate sub control, you should really stop and ask yourself if you actually DO need it.
Here’s the thing. When you add subs to your PA, they will either be balanced by the manufacturer (reason why you want to get subs matching your full range speakers from the same vendor) and/or have their own volume control. Why the latter? Because the low end is most susceptible to the environment. I.e. depending on the venue a certain sub volume might be too high or too low. You use the volume knob on the sub to balance your set BEFORE the gig starts and pretty much leave it where you set it for the rest of the night. In case you find at some point that it needs a (little) adjustment, you can easily walk up to the subs, make the adjustment and get back behind the desk.
I don’t believe in EQ’s on speakers. There is a reason why all but the most high-end speakers with digital DSPs in them don’t have them.
Also the sub should take part of the frequency range from your full ranges (the bit between the lowest on your full range, usually around 55-60Hz or so, and the high end of the sub which is typically in the 100-125Hz region). This is the part that draws a major part of the energy in your full range and gets freed up to do the mid-low and mids better. If you keep your full ranges full range (duh), you will be losing that benefit and in return add to the 55-100 Hz energy your PA produces, thus messing up the intended balance. Your sound will sound boomy and lopsided and your mid-low/mid will sound underpowered by comparison.
So, while it’s a good plan to have a small (6-channel) PA mixer (Yamaha MG-series, Mackie and a few other brands) for various reasons, the reason you state is not that reason. A good reason could be because your controller only has RCA master out and you want to go balanced XLR/Jack to your active speakers for example.
At the end of the day you should have one or two subs (depending on various factors) with two top speakers, preferably same brand/series. You should run balanced cables into both subs (typically the subs will come with the crossovers), then from the subs to your tops. If your controller has balanced out you don’t need a mixer, if it doesn’t either an unbalanced-balanced converter (ART has one) or a small PA mixer will do that trick for you.
Final warning: do not run your subs without a crossover! While hardly audible (the big magnet and cone of the sub can not move quickly enough to properly reproduce the high frequencies), the highs will fry your magnet if you put too much volume on them!
.
