Weather changes are causing the distances between your cathodes, grids and anodes to change. Either that or your inner ear properties.

I've noticed when I turn my fireplace on and my multichannel room heats up, my audio quality goes out the window. For whatever reason, I don't have that problem with my 2 channel system when the furnace is on.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated