Electrical power related??
Will need a little more info from you.
* How many dedicated lines? Are you sure they are dedicated and not multi conductor separate circuit lines?
For a typical single phase service found in a residential dwelling a multi conductor branch circuits can supply (2) separate 120V circuits with (1) common shared neutral.
With a multi conductor branch circuit the voltage on each 120V hot leg to the shared neutral will be 120V nominal. (From one hot conductor to the other hot conductor, 240V)
Only the unbalanced load of the (2) separate 120V circuits will return on the common neutral conductor to the source, the electrical panel.
The balanced load of the two circuits are in series.
If for any reason the common neutral conductor becomes open or loose it cannot carry the unbalanced load back to the panel. The voltage from each hot conductor to the faulty neutral conductor will not be stable and will swing wildly with the loads on each hot conductor. Just to pick a number, the voltage at the receptacle feeding the preamp could shoot up wards of 150V. The voltage at the receptacle feeding the power amp could drop to 90 volts. The greater the load imbalance the greater the voltage differences.... In your case it could be a poor connection on the return common neutral conductor.
* Have you been having to replace a lot of 120V light bulbs lately?
* Have you noticed lately some lights seem to get brighter when the refrigerator or some other appliance turns on?
The scenario I described above with the multi conductor branch circuit is the same for the electrical service that feeds your home from the utility transformer. You have (2) hot conductors and one shared neutral conductor. If the main service neutral conductor has a faulty connection, high impedance, this will cause 120V nominal voltages on the hot service conductor with a larger load than the other hot service conductor to have a low voltage, and the other 120V of the other hot service conductor to have a higher voltage.
The poor neutral connection could be at out at the street at the transformer.
If an overhead service it could be at the weather head where the power company line connects to your line.
It could be in the meter socket.
It could be at the main electrical panel.
At any rate you would need to hire an electrician to check things out. He can check voltages at the electrical panel and tell if the problem is a poor service neutral conductor connection....... He can also check the branch circuits that feed your audio equipment.http://openbookproject.net//electricCircuits/AC/AC_10.html#xtocid81990