+11. it's the whole band's problem to fix. ex. drums should change up more, guitarist should solo better,
There's no one size fits all solution, but it always starts with the whole band and starts BEFORE the solo. The guitar player needs to understand how to play in a three piece band and that means you don't blow the doors down with big fat chords and then hit the solo and disappear. The more the guitarist understands this, the less there is to "fill in" during solos. The drums and bass need to hit the groove a little hard and I think it helps if the bass simplifies, not throws more notes at the problem. Plus, learn to embrace the space. Too many bands throw a wall of sound at the audience all night long. If you do all that first, there is less that needs "fixing" with effects and they will be more effect-ive.
I also like the idea of a signal splitter. Maybe send one signal to an octave pedal into a guitar amp to subtlety fill in when appropriate.