Ground loops are a real problem. You really don't think about it until you get the problem. Then, it can be a bear to resolve. The MOST common way to create a ground loop problem is by installing multiple dedicated lines. You think you are doing something good for the performance of your system - but as it is with many things, the more complexity you add to a system, the more likely you will have unforeseen side-effects.
When doing a new installation I usually recommend a maximum of two dedicated lines be installed. One for the source components and one for the amp(s).
What is most important, no matter how many lines you install, is to over-rate the dedicated lines and pay particular attention to wire gauge, conduit, outlets and the breaker in the electrical panel.
If you have a simple system (3-4 components) and don't have massive power amplifiers, one dedicated line is sufficient when done correctly.
Only when you have massive or multiple amplifiers (home theater) do I recommend more than 2 dedicated lines. There are some amps like PASS, Macintosh, Boulder and some class A amps that pull very high current loads and MUST have seperate dedicated lines to perform at their true potential.
If you are using solid state amps with truly balanced type inputs and outputs, you will not usually have any problems even with multiple dedicated lines. And if you do have the problem with this configuration it is easy to fix.
The problem most commonly occurs when using single-ended interfaces to connect amplifiers that are NOT on the same circuit as the source components. JFYI, all RCA type connections are single-ended and even many XLR type connections are actually single-ended not true balanced connections. And the problem is more often found when the amplifiers are tube types.
Just to be clear about this: there are many other causes of ground loops even if you only have a single dedicated line. So this is not meant to be a comprehensive discussion on ground loops. This is about dedicated lines and why you may NOT want to put in too many of them. And when you do - pay attention to the details.
Ground loops are caused by differential currents running through the ground wire (return) in the interconnects. There will be a voltage difference between the signal grounds between any two components. You can easily measure this with a simple volt meter. When different dedicated lines are installed, there can be a difference in potential (voltage) from one ground connection to the other relatively. The best way to avoid this is to ensure that the impedance of the ground connections are the same. This means using the same type and gauge of wire, the same length, no splices, no junction boxes, the same outlet type and ensuring good reliable connections in the electrical panel. And the electrician should not coil excess wire in the wall! It is not just about the length of the wire - it is total impedance that is important.