Turn it up...?

Are you measuring with a meter?  It is merely conjecture otherwise.  Good thread starting material......

Typically the noice in power is much lower at night… so, the distortion and high frequency noise is lower. That can sound like lower volume… blacker background. 

Hmmm...my space runs 'bout 24 dB typically...quiet enough that the calibrated mic on the RTA picks up me clicking keys....(5ish' ft., 95 deg. out).

Cell ap, fairly popular...

Daytime is subject to 'the world outside'....*L*

@mapman     Agree.  Need to measure sound levels before jumping to subjective conclusions.

@imhififan      Unlikely voltage variations can cause this.  In UK +-6% variation is the legal maximum.  Most amps draw power and store it.  Once there is sufficient in store the amp will perform consistently.