I’m looking for opinions on those who have used both, if you could comment on any differences between the two? On the tin they seem to be very similar. Does either one have strengths / weaknesses vs the other?

With Zynaptiq having a rare sale on unfilter this month (may), I would love to get a discussion going before it ends.

+1 anyone have and often use both? ..(and DSM?)

Good question same price at the moment....

I use Gullfoss all the time, sounds great, really easy to use. I did demo Unfilter, but didn't like it very well. Maybe I didn't know how to use the various controls, but it sounded overhyped in the highs & low end. Gullfoss sounds more flat in response, which is what I prefer. Does a great job of unmasking frequencies and making things sound cleaner without resorting to using lots of additive EQ.

The only thing the same between Gullfoss and Unfilter is that they both offer some kind of automatic EQing.

Gullfoss works dynamically in real-time adjusting its EQ curve based on audio input. There are controls to adjust the nature of the EQ algorythm to bias its adjusts one way or the other to suit the input and what your brain feels is pleasing. Gullfoss is designed to work generally across audio inputs dynamically and not solve a particular problem other than making the audio sound better over-all.

Unfilter "learns" the nature of the input and then applies a "transfer curve" to it that has some parameters for how that curve is to be applied. However you can also run Unfilter in dynamic mode by leaving LEARN on. I assume they call it a "transfer" curve rather and "EQ" curve because there is more going on in the code and curve is used to determine how. After you "learn" the input audio it is a static process based on parameters that control the nature of the algorythm. Unfilter is specifically suited to "unfiltering" audio that may have been filtered by room resonances (causing comb filtering) but Unfilter also acts to generally spruce up the audio in static way using the transfer curve.

I use Gullfoss to help clean up the spectrum for music sources in general, specially in mastering to nab any smaller issues I couldn't solve with a manual EQ. It works well for that. I find Gullfoss more transpartent than Unflilter in those applications.

I use Unfilter for more problematic audio that need help, such as poorquality live recordings and restoration work in general. I use Unfilter more for restoritive work or to add "spritz" to other audio, it's not a subtle processor.

There are a couple more I use in this realm, Acon Defilter and oeksound soothe. Defilter is more like Unfilter but it has a different sound to it, so I use which ever one works best. Again, these are mostly for bad sounding audio restoration, specifically audio with resonances and combing. soothe is more like Gullfoss but is only centred around resolving resonances. It uses some kind of spectral processing that has it's own sound giving soothe a specific character different sounding than just an EQ curve. soothe is a good general purpose audio cleaner-upper for tracks and busses but I find it a bit heavy-handed for mastering.