I would suspect it was not “the toy” as the MC3000 is a professional grade controller. Sound quality on all Denon controllers I can vouch for is top notch and used by many professional DJs daily.
So I would suspect the problem was in your signal chain.
Things that can cause the sound to water down:
The Pioneer Mixers (especially the 800s and lower) sometimes the Line/Phono switch has issues and get stuck on Phono (for turntable use) or actually are reversed (Line is now phono). Of course if you connect a controller to a phono input the sound will suck and be low volume at best.
Next the 800s and lower have the input RCA soldered to the board inside and not screw tightened. So the inputs may simply be bad.
The EQ section of the mixer could have been faulty or “stuck”.
The gain setting on the mixer could have been wrong and the software output was too low. (Signal chain has to be 0db at sound check)
There are more things that can go wrong, but these are the first that come to mind.