Serious dynamic range problem using USB audio interface on ALL Pioneer hardware?

FYI: dynamic range is not the same as digital audio level.

There are ways of finding out exactly what the issue is, why, and demonstrating exactly by how much levels are out of whack, but since you seem to be unsure and unspecific, you'll just have to loosely troubleshoot this as best you can.  The software and the mixer with built in interface should both have some settings to change things, and then there's also the software's visible trim/gain knobs.  Check all of them, try changing things, and experiment to find what works for your arrangement.  If it was me, I'd be sending test signals, checking their levels in the mixer, looping back to the computer to check there, and doing it a regimented manner.  That way, I can definitely say what's wrong and prove it to others if I can't fix it myself.

Unspecific? Ok here we go.

DJing with rekordbox in performance mode, using a nexus2 set (cdjs 'sand cdm) with ASIO drivers on a windows 10 laptop, using the mixer as sound interface in external mode, will clip internally in rekordbox.

This happens without using effects, autogain or any volume changing parameters, most tracks will just clip before they even arrive at the mixers channel. To fix this you are forced to change rekordbox' output level and set it to either -6 or -9.

Can you explain why playing with any other brand USB interface doesn't clip internally before the audio leaves the laptop, but with a Pioneer device it does?

Again, there are no software gain/trim knobs, I am talking about external mode.,

Do you know what external mode is?

If you're so certain what's going on and that it's not the Pioneer hardware where it's clipping somewhere, then just do an in-situ PC test with a free computer scope program and demonstrate a calibrated test signal exiting Rekordbox in a clipped state without any software trim/gain alteration, EQ, effects, summation, or alteration by the external hardware.  That's easy enough to do and prove to Pioneer DJ what the problem is so they can fix it.  Post pics or to a YouTube video of your results.  In the meantime, prior to a fix, if that is indeed what's happening, running at -6dB inside Rekordbox is a perfectly reasonable mitigation considering it's just a bit-perfect halving of the signal levels.  Usually, you'd add headroom for internal software mixing mode, but it's certainly not any more destructive for external mode than what's happening inside the DJM.

Problem also occurs in other software and on the new DJM-V10

https://www.reddit.com/r/PioneerDJ/comments/ux9g4o/djmv10_clipping_with_serato_dj_pro_usb_output_too/

And here the same problem observed on a DDJ 400:

https://www.audiosciencereview.com/forum/index.php?threads/ddj-400-measurments.29480/

And no, -6 dB in rekordbox is not a perfectly reasonable mitigation, I should be able to play an MP3 file "as is", using the full dynamic range.

In many cases this issue led to problems. For example DJing after a DJ that played with USB stick, on a system that is mostly used on max volume with master leds on the mixer in the red.

If you try to DJ after this person that used a USB stick, you will have a hard time even reaching the max volume (channel gain would be fully open).

Also DJing like that just sounds a lot more crap then when your audio signal isn't severely lowered from the start. I now no longer use the Pioneer DJM USB interface when I DJ in performance mode, I use my Apollo Twin connected to the mixer with analog cables because I was tired of my music sounding crap on big systems.

Why has this been going on for years?

It is a perfectly reasonable mitigation, as even changing the pitch or key or scratching will raise the levels slightly, and -6dB is the minimum nondestructive way of padding the signal.  Within the domain it's operating in, there is no loss of fidelity from a pad like that.

The V10 thread was resolved when the user realized Serato's master volume in external mode was still affecting the individual channels exiting the software.  The idea that the Pioneer products are altering DJ software's intrinsic gain structure and thereby causing clipping in the software apart from any volume controls is hard to believe.  Either it's running up against the wall inside the software because some setting is goofed, or it's having problems later in the chain.  While it's possible your DJMs are telling Rekordbox to change its internal gain structures somewhere from some MIDI or HID instruction, like that Serato master volume, you've already claimed this happens on other software.  I'm still not convinced it isn't happening in the Pioneer ASIO utility in Windows or the DJM itself, like one of the mixer's USB input levels settings.

The DDJ-400 measurements thread isn't relevant here because it's using Rekordbox internal mode and thus requires substantial headroom, just like every other DJ software & controller scheme... or digital DJ mixer without DJ software, for that matter.

BTW, your CDJs have ASIO interfaces, too, but if it were me I'd just be dragging and dropping files from Rekordbox file management to the players using Pro Link and not be screwing around with Rekordbox performance mode when you've got separates.