I'm rather convinced that it is my interpretation of sounds that varies.  With the amp in the same room and with the same guitar, settings, etc. some times the tone is very pleasing and other times I'm making adjustments.  I think it's similar to how our tastes for the same foods can vary. 

I am a medical doctor and had to learn in much detail about our mechanisms for hearing and the neurotransmitters involved.  When you hear a pleasing sound from your guitar and amp it gives you energy because certain neurotransmitters are released (in particular - dopamine).  How many times have you played late into the night and are not tired?  This is similar to the good feelings that long distance runners get with production of enkaphalins and endorphins.  When dopamine and other neurotransmitters are released our bodies need some time to replenish or recycle them.  This is the primary variation that affects how pleasing our setup sounds at the time. 

Incidently, the street drugs that are addictive involve these same neurotransmitters.  After taking methamphetamines, for example, one's dopamines supplies are depleted in about a day or so.  Then this person feels terrible and can only think how much better it would feel to be on the drug again.  I play guitar, listen to music, and exercise because I enjoy it and I feel good in a healthy way. 

Wall voltages are important. I always wanted to try Furman power regulator/conditioner but never had a chance. It is very expensive and I don’t want to spend 1000 pounds on it before trying it properly.
Is anyone using it? Your thoughts please.
Only variac won’t work because it will change voltage up or down depending on wall voltage change. 

I got the impression from Boldaslove's comment it does work?

Very true. In *most* cases it would need to be adjusted up at most venues due to the load of all the equipment at the venue

I bough this cheap power conditioner, it's working fine and good thing about it is, that it has this voltmeter on it...



...the idea was, that with volmeter I can at least follow the correlation between the wall voltage and tone I have.

...but the jury is still out on that one... I think that ear fatigue or even the way I feel at times, has more to do with that.
...but what do I know.
 

Does this thing keep the voltage level at 220? or does it only show the voltage?

I saw there's a Phonic 9000E. Is that the same?

Sorry for my ignorance but I thought the whole point was to even out the voltage and making sure it always is 220V?
I sometimes feel voltage drops changes the sound. But I must confess this is based on knowledge just on gut feeling.