Does your system sound better in the winter?

Yes, I have noticed better sound quality in the colder months. Here in Houston I still listen frequently in the summer, but with my heat generating all tube system I can cool the room simply by opening the windows.

Phild you might consider a Tripp-Lite LS 604 line conditioner ($138.65 with $10,000 insurance). This device will bring voltage as low as 87 volts up to a minimum of 107 volts. A maximum of 140 volts down to a maximum of 126 volts.
Don't risk ruining your stuff from a brown out.

Could it have to do with the slightly denser air at the cooler winter temperatures? Since sound doesn't travel in a vacuum, going towards the other extreme could mean better transmission of sound. Seasonal humidity differences might also make a difference.

Or maybe your earwax is softer in summer. Or maybe your outer eardrum cells are drier and less pliable in winter. Or maybe the coclear fluid in your inner ear is slightly more viscous with the cooler temperatures. Or maybe when it's hot you have different bloodflow patterns in your ears. Maybe snow and ice act as ambiant sound absorbers or diffusers.

My best guess is that it's just generally quieter and your equipment likes the temperature better.

It is not only the summer noise. At least in my case, the power quality is worse in the summer. All summer I have had to endure a persistent and loud buzzing from my system. It is so loud I really can't listen to it during the day. And no, it is not a "ground" problem. From the hours of 2:00 AM to 5:00 AM my system is dead quite. It is also quite though the rest of the year.

I hate summer!

Check out a pair of Richard Gray Power Supplies. I was very leary of this product's claims but was offered a no questions asked return policy from my dealer and gave them a try. It dramatically improved the sound and for the first time my system sounds consistant at all times of the day. That said I live in an 1920's building with old wiring and lots of neighbors so ymmv.