There is a raw chewiness to the phasing. Part of this is because the FX20B is a six stage phaser, so it has a wetter sound than any four stage phaser. The other part is due to its low input impedance. DOD listed the input impedance as 500Kohms. In reality it’s only 100Kohms. If your guitar is running straight into this pedal there will be noticeable high end loss. It’s like your guitar signal was going down the high pressure pipe of your guitar cable and landed in a pool of cold jello.
Unfortunately the impedance issue is there even when the pedal is bypassed. Luckily If you have a buffered pedal somewhere before this FX20B the impedance issues shouldn’t matter.
The actual phaser effect isn’t terrible, so long as you aren’t expecting it to be something it’s not. It has quirks. The phaser sweep spends more time at the bottom than the top. The non-symmetrical sweep also seems to rise more slowly than it falls. The eccentricity is especially noticeable at slower speeds.
The dark tone that helps it cooperate with gain is not as friendly to clean signals. It’s definitely better suited for brighter guitars and single coil pickups. With humbuckers there’s little high end swirl and a whole lots of viscous low end goopiness. Unfortunately it’s less like a hot fudge sundae, and more like cold 5 day old leftover tuna casserole. Mmmmm just like mamma used to make. (Sorry Mom, that stuff was terrible.)
The controls feel weird. Especially the depth knob which also raises the phaser sweep. This is an oddity I’ve never experienced with other phasers. On some phasers, along with a depth control, you’ll also have a manual or frequency control. Turning the depth knob up on the FX20B is like turning both controls up at the same time. The truth is though, you’ll probably find yourself cranking the depth setting and wanting to break the knob off. There’s really no other setting for this knob.