tarp tone?

im not sure if this falls under bass or amps...could be either

the outdoor covered stage that we played on sat has drop down tarp walls for when it rains or is windy or both. weve played there many times so im used the sound/acoustics of the stage without the tarps. the bass i was playing is a warmoth jazz V, emgs (VVT), maple neck, rosewood (i think) finger board, not sure on the body...alder, ash or maple...strung with black beauties. this bass normally sounds scooped to me, big bottom, zingy top, weak in the mids. i am able to i roll the neck P/U off maybe 15% to 25% and the tone control back 40% to 60% depending on the tune and get the tone to where i like it. we got rain delayed and when we started back up i noticed a tonal difference. with the tarps dropped (about 3 to 4 feet from my cabs, 2 eden 212xlts) the tone got thicker....more mids and low mids...i was a happy camper. i didnt change anything on the bass or amp.

am thinking the tarp absorbed some of the freqs or reflected some/other?
or could the humidity have effected the bass that quickly, that much?

anyone run into something like this before?