Effect during guitar solo

[Oreomeister365]

Would def say fuzz. Maybe with a blender to retain some fundamental bass signal (so it doesn't sound like your bass disappears instead)

[Floyd Eye]

Brutal fuzz, such as TAFM or Malenko B:Assmaster

Octave pedal.

[Dark Barn]

I don't think you want to do any one thing for every solo, I think you'd do best to switch it up, sometimes using effects or technique or both to add volume and interest to the guitar solos. Also your guitar player should look at ways to beef up his tone during solos.

As a listener it would get old to always hear the same effect, especially the octave ones discussed here (Unibass, Fission, POG), kick in for every solo.

[Bakkster_Man]

I think the idea to change it up is a good one. Really depends what the song calls for. Listen to various 3 piece bands and what the bassist does in solos.

For example, for a slow jam I'm thinking heavy phaser like Ted Nugent's Stranglehold.