I had recorded a double bass for a track, and the bass had some resonances which needed tamed. I tried the usual techniques I use on bass guitar (multiband compression), but the resonances moved and changed with each different note being played. So I thought, maybe Surfer 2 EQ would work? It tracked the fundamental perfectly, so I could bring that under control, just using a small cut. The second harmonic made the bass sound wooly and boxy, so I tracked that and reduced by a few dB. I accentuated the 3rd harmonic to bring the bass out slightly and finally around 1.5khz a small boost to add snap to the notes. Once I did that, then using a multiband compressor to level out the volume worked perfectly and the bass finally fitted into the mix. I've so far used it on bass and vocals and it's fantastic, doesn't squash the dynamics, but allows you to control things. If you overdo it, it does produce artifacts, but I only use it with small dB cuts or boosts, which keeps artifacts to a minimum. It's going to be interesting seeing what other solo instruments this works well on. It's a plugin that works on solo instruments, because it can track their pitch. If you try to use it on polyphonic instruments it's not suited for that.
