I use the Avalon U5 very regularly (hi-cut always on.) With active basses I often leave the Avalon tone circuit off with the boost around 3 or 4, and adjust the on-board eq to taste. However, if you want to use the Avalon tone circuit, I would suggest playing your bass in passive mode into the U5, boost around 4 or 5, and try tones 3 (mid-cut) and 4 (mid-present) to find which sits in the mix the best. You can then adjust your passive tone knob to find the right amount of treble freq's you want.
I absolutely love that pre, but it's not for everyone. It's VERY clean, and IMHO very fat too. I think the boost has a sweet spot between 3 and 6, but that may just be perceived volume affecting my judgement, or just the appropriate level to send into my compressor (which I only use live), but IME it gets harsh at 7+ and weak below 3. Some people think the eq is too dramatic for bass, but I think they can definitely be used to your advantage, depending on the weaknesses of the instrument (shouldn't apply to a Lull,) the room if used live, and also the density of the mix (in your case that pianist's left hand.)