Passive Pre-amp: Improves overall sound quality??

I will answer part of your question. The main circuit of a passive preamp typically does not require a power supply. So no AC power required. In some cases, such as those using Light Dependent Resistors a power supply and AC power is required. You may also require power if your passive features remote control volume like the Placette RVC and no longer available Bent Tap (although I think you can still get the DIY remote kit). The idea and one of the main attractions of a passive is simplicity and to act just as an attenuators and possibly input switcher.

Now for the rest of your question spend the next several days reading this thread and I think all your questions will be answered:

Lightspeed Attenuator - Best preamp ever?

When it comes to active preamps Ralph is the only designer I know that adds one more item to the list of what an active preamp should do and that is control the interconnect. As someone who is very passive friendly I can respect this point of view. If an active preamp is "acting" as a tone control then it is a flawed design or designed to perform that way, which in my opinion is wrong headed thinking, or it cannot control the interconnect and its artifacts. Transparency should be the key attribute of any preamp.

BTW I currently use a passive with my analog set up (tape and vinyl) with no issues. That is soon to change though.