"For example, volume at "25" on the pre-amp would normally be quite loud, but now it needs to be turned up to 40 to get the same loudness". Also, the description of the vocal midrange disappearing into a recess.
This is exactly what I have experienced with equipment where the internal power supply is vastly under-powered (i.e. not enough capacitance after the power supply regulators). What happens here is that it starves the audio circuit and you feel like you have to keep turning up the volume to hear stuff (but you never quite get to the point where it is satisfyingly loud enough).
Obviously, something happened to your electrical circuit when the guy fixed the light. Electricians do have a tendancy to want to chain outlet after outlet. It could be that he tapped into your audio circuit somehow and it is now being chained after several outlets/light fixtures. The idea of a dimmer knob on a light fixture is an excellent idea. Dimmer switches do not turn down the actual use of electricity. They just shunt it through a variable resistor so that it limits the amount of electricity/voltage going to the actual light bulb. This could be playing havoc with the voltage supplying your audio equipment. I would take a voltmeter and measure the voltage of your audio power outlet to make sure you're getting at least 115V. More does not hurt and 125/130V is fine for audio equipment (though some preamps can sound harsher with higher voltage if the internal power supply is not filtered as well). A higher voltage can have small benefit as the audio power supply as it allows a higher level of voltage capacitance in the main power supply before the voltage regulators step down to what's required in the audio circuit.
I believe your audio equipment is being starved by either voltage or current.