FabFilter

I'm very interested in Pro-Q's ability to dynamically boost frequencies, especially now that it can be done on a spectral level. But what stands out to me is that the dynamic adjustment can only be triggered when a sound goes above the threshold. If I wanted to use this feature to boost an area that I want to be more resonant, this is counter intuitive. I would be much more interested in Pro-Q 4 if an option were added to switch the threshold mode to upward expansion where sounds that fall below the threshold get boosted instead. In the meantime while we don't have this feature, my best workaround will probably be to trigger dynamic bands with external sidechain, or find another plugin for the job.

If sounds that falls BELOW threshold gets BOOSTED that's not upward EXPANSION, but rather upward COMPRESSION.

Compression is when you reduce dynamic range.
Expansion is when you increase dynamic range.

You have upward expansion in ProMB.
You also have (sort of) upward compression in ProMB, if you push "range" in Compress mode into positive values and lower your threshold, compressor will stay "down" while above threshold and boost when below threshold.

Pure upward compression is pretty rare in plugins. (and i hope it gets implemented to ProMB and ProC eventually)

You won't solve that with external sidechain.

ok, then what I meant and what I want is spectral upward compression. That's good to know that Pro-MB has that, but that's still not spectral shaping, which is where I want to apply this.

MSpectralDynamics by Melda is the only spectral processor capable of upward spectral compression.

I'm pretty sure it won't work as you'd expect tho, upward compression is more often than not noisy and odd. Can be great when used sparingly, but you need good source to begin with.
Not sure what you're trying to accomplish tho.

Anyway, it's currently on sale for 90€ and you can do a 14day trial to see if you like it and does what you need to do