dvdhawk may be correct.
With no schematics or block diagram for signal flow, it's difficult to know for sure.
BUT, there is one clue in the manual. It is the description for the Send Out (exactly as written in badly-translated English):
"18) SEND out-this is a 1/4" 2-conductor line output. The impedance is 1k ohm @-10dB. it is used to connect the either additional power or as and additional Recording device. This line level signal is the mirror image of the mail outputs."
Well, I suppose it IS possible that they chose a completely stupid place to put an Effects Loop....or they just don't know what they were writing?
IF it is a "mirror image of the mail (main) outputs", (which I would guess they mean 'duplicated image', as a 'mirror image' would be reversed), then THAT means it's even post-master volume, since the Main Outputs are that.
And, if THAT is the case, then there IS no place to inject the signal back into it. SO...we have concluded that the Chinese don't know how to write a usable manual. A loop AT the MAIN OUT has no place to get back in!
I think dvdhawk may have been saying that it could be post Master EQ (the 5-band graphic) but pre-amp. Which is STILL a completely stupid place to put an effects loop, but who knows what the Chinese are thinking when they design those things?
The more logical place to put one (that far into the system) would be right before the Master EQ (5-band graphic), so that whatever comes out of the 8 channels, submixed, can be dealt with (effects and eq) separately from the mains.
So...it's still a mystery about EXACTLY what that thing does, where. Lacking a block diagram or schematic, the only way to know for sure would be to connect it, and see if the Master EQ and Master Volume affects the Send Out (of which the manual insinuates that it does). Which doesn't make any sense, if it does. Or, if the Master EQ affects it, but not the Master Volume. Which is only SLIGHTLY more sensible, but still stupid.
One thing I'm pretty sure of is that the manual sucks!
I'd just experiment a bit. And...to reiterate....no matter how it works...don't power up ANYTHING until ALL volumes are DOWN! And, then...work from there gradually. If you can get the Pyle turned up quite high in the Main output, and the channel volume on the Pyle fed from the Mackie reasonably centered, and it's still clean...that may be the best you get. But...you'll probably have to do a lot of small adjustments ( a touch less here, a smidge more there) at any point that can manipulate the output levels of the Mackie, and anything that can affect the levels in the Pyle...input to output....to keep hiss and distortion as low as it can get.
Kapt.Krunch