Hello Renoise forums. It’s been a while!
I’m about to lose my mind here though. 
Somewhere within Renoise, at least as far as I can tell, there is some sort of built in compressor or limiter.
As I turn on or off different sounds, or enable or disable the line in from my keyboard, the rest of the sounds get quieter.
I’m sure I’m not imagining it, in fact I think I can see it happening in the top vumeter. I have a big bass drum droning over some orchestral stuff, and if I enable/disable some rather loud hihats (I’m just setting up the sounds for this song) then the volume of the bass drum and orchestra changes dramatically.
Please tell me I’m not going crazy! If the problem is in Renoise how do I turn it the h*ll off?! If it isn’t in Renoise, could it be some unwanted part of Windows 11? I’m using digital out to an amplifier and I can hear it in headphones as well as speakers.
Oh my god…
It’s the amp. 
I thought I would just have one last look at my amplifier settings before sending, and there it is. Dynamic Range Compression. FFS.
Oh well, I’ll still post this just to say hello, to prove that I’m not crazy, and in case it saves someone else spending 4 hours pulling their audio chain apart.