I posted this in DDJ forum too. Mark said he is looking into it. I'm not sure which place will get the attention it needs to be resolved & it's a bug/suggestion, so I'm posting here too.
I think I found a clue to the problem. The Computer Audio Output to the Controller Audio Input has a channel change when Serato starts running.
With Serato running, the Driver Utility changes USB 9/10 from "Sampler Volume" to "Cue". The audio connection for the Sampler is changing when Serato is turned on. I don't know why "Cue" would need its own input, since it just listens to CH1-4 & the summed Master anyway.
Without Serato running but with DDJ-SZ2 connected, USB 9/10 says "Sampler":
With Serato running and DDJ-SZ2 connected, USB 9/10 turns into "Cue":
If Cue output is really needed, I would think it'd be needed for using DDJ-SZ2 with other software (like Traktor), rather than with Serato. I'm wondering if this wasn't a boolean logic mistake with someone writing a true/false IF statement. i.e. maybe it's backwards.
I can't imagine a case where you'd want to use a "Sampler" channel with DDJ-SZ2 connected to the computer, but not connected to Serato. It sounds awfully specific to Serato. So that's why I'm thinking it may have been reversed in the Driver utility.Can you make it so that it sends the Sampler audio separate from Channels 1-4, while running Serato DJ?
I learned that it is copying the Sampler volume across all 4 channels (not dividing it by 4 and then copying, it's just straight copying), when using Master output. The result is a master sampler volume 4x louder than it's supposed to be, when all 4 faders are open.
Since I mix with all 4 channels, the only option right now is to keep it in Master output and be careful to only have one channel open at a time. It's worsened by the fact that each channe's trim also adjusts the gain of the Sampler on that channel... so if your music track is loud & have trim down low... your sampler will be quiet on that channel until you raise the trim again.
Please address this ASAP. Your DDJ-SZ2 is a great tool for serious DJ's that want to be very mobile. Unfortunately, these extra features like Sampler controls are some of the reasons we go for a controller instead of a CDJ setup... or a cheaper controller.