Mira

It's mostly ear fatigue plus loudness drift, and no, it doesn't fully stop — but it becomes predictable and you build around it.

What's happening: over a three hour session your ears desensitise to high frequencies, so you keep adding top end and it keeps sounding fine. You also creep the monitoring level up as you go, which makes everything sound better in a way that has nothing to do with the mix. Then you come back rested at a normal volume and hear it honestly.

The fixes are boring and they work — mix at a fixed monitoring level, take ten minutes off every hour, and make the last decision of any session 'save and stop', not 'one more pass on the vocal'. Also, harsh and over-compressed specifically is what a tired person makes, because both make things sound more exciting to fatigued ears.

Mark your monitor knob with tape at one position and never move it while making EQ or compression decisions. It feels restrictive for a week then you stop noticing.

Did the sweep. There's a very obvious dip around 110Hz right where I sit and it's much stronger a metre forward. Moving the desk this weekend.

One more: check how much low end is coming from things you're not thinking about. Room mics, piano, pads and even reverb returns pile up under 100Hz. High-pass everything that isn't kick or bass at 100Hz and the low end often clears up before you touch either.

Also possible: phantom power pop or a handling thump hitting the preamp on a transient you can't hear because it's below 30Hz. A high-pass at 80Hz on the way in won't help the preamp, but a shock mount and a solid mic stand might.

Whatever you buy, spend an evening first on positioning — equilateral triangle, tweeters at ear height, symmetric distances to the side walls. That's free and it's a bigger jump than most £350 purchases.