E string sounded like E flat before gig

[ultra60]

Gig last night in a room I've played before with same amp, etc. Bass was tuned but when I plugged into my amp, the E sounded flat compared to the other strings. Guys in band noticed it as well. Checked bass several times and perfectly in tune. Guitar player plugs in and same issue. His low E sounded flat. As we played together, things sounded fine.

I don't recall this happening before. Any theories etc? We had not started drinking yet (lol) so I can't say that was a factor.

 

[Ggaa]

Ime some rooms/stages do that, correctable w/mid eq when setting up. Parametric is good for that. Most of the time it clears up by sweeping downward, surprisingly rather than up on my rig, anyway, ymmv.
Sometimes when that happens you can move a fret either way and not hear a change....