When I first started playing fretless I used open strings wherever possible so that I could use it as a reference pitch to determine whether I was in tune or not.
As I got more experienced I substituted the open strings for fingered notes for consistency of tone, but it's an exercise that served me well.
Bear in mind though that you're in a different position relative to your amp when someone else is playing, and high frequency sound is directional, so it could just be that.
Great bit of kit, I love mine! Using it with an expression makes it ten times more useful, as it can be set up to control multiple parameters at once - e.g. increasing the gain on a distortion and lowering the output to compensate, changing the rate of chorus or phaser etc…
It also serves as a split - point for separate feeds to a DI and amp as it’s a stereo out!
Every home should have one.
I use a Line 6 M5 with an expression pedal for modulation, delay and pitch-shifty weirdness; everything else (filters, compressor, octave and many, many filth boxes) are honest-to goodness analogue units.