I am having the very same problem with the DDJ-SZ2 using Serato. Pioneer, please reply! This issue has been ignored by Serato and Pioneer from what I can see. Not a good decision for your Professional grade controller.
I have my Sampler output set to "Master" in Serato, which their website says goes to "Master Mix output". However, it's very clearly being split into 4 channels (you can see all the meters go up in the green when you press the sample on max sampler volume).
The master output level is amplified by the number of channels open (so open 4 faders = 4x the volume of 1 open fader).
This is terrible because the Sampler should be a 5th "Aux" channel layered on top of the main 4 channels, not split across them.
I haven't noticed this issue with my previous DDJ-SX2 or DJM-S9, but I heard the DDJ-SZ had this issue also.
Please fix this issue, so the Serato Sampler is output to the master channel. I notified Serato of this issue also, but it's still a possibility that it's a Pioneer driver/firmware/utility software issue.
Please investigate alongside Serato & fix this for your professional serato controller.