Even if the line is "hard" at your transformer, your 100' drop coming to the house may be too small a wire gauge or may have bad connections, resulting in excess voltage loss during high current demand (heat pump running). Use a known-calibrated voltmeter to measure both 120V legs incoming to your distribution (fuse or breaker) box in order to find out. Comparable measurent at the load end of your dedicated audio circuit won't read much, if any lower, due to that large #8 gauge wire. But if you can access any measurement points right after the transformer, before the power enters that 100' drop cable,(being very careful, or better, having the power company measure this if you're unsure regarding safety issues) then comparing the at-house voltage against the at-transformer voltage will tell you if that 100' drop cable is too small. Or it will also tell you if the voltage out of the transformer is too low. In either case the power company has to take corrrective actions. But typically they won't do anything for you unless you supply measured evidence of a problem. And even at that, you'll probably have to make your valid complaints known to an account management supervisor before they'll do anything about it, based upon my experience with the glacial speed of power providers. Their way of thinking "if the lights are on then everything's fine" has been my unfortunate experience.
Electricity fluctuations
I live close to the end of the lines at a rural set-up, I have 100' underground cable coming to my house. I installed a 20amp circuit and an 8 gage 35' dedicated line to my stereo. I get current fluctuations that I could not detect through the voltage indicator of my PS 300. I run various air cleaning filters in the house and I hear them slowing down and accelerating intermitently but what bothers me is that I can't use my turtables' power supply (VPI, SDS) at the recomended lowest voltage of 72 volts because the motor stalls. The power company tells me to check the cable that comes in my house but then when I operate the turntable late at night, after 12pm it runs okay at 72volts. It is mostly during peak hours that it stalls. The SDS is plugued in a PS 300. Can anybody tell me what to do to corect this problem? I am handy but not too technically educated.