Passive Pre-amp: Improves overall sound quality??

This topic has been discussed at length in the forums more times than I can count. You might want to take a look at what is already been said. At a high level, you will note that people have some passion around this topic. There are different types of passives. Resistor based, transformer based, autoformer based, and others perhaps. These tend to be zero gain designs, so that all they can do is attenuate the volume from the source. They do not amplifiy. Resistor designs burn the extra volume off as heat, whereas transformers work by decreasing voltage by increasing current. You need to do some work to ensure that your source has high enough voltage to match the input sensitivity of your amp. Otherwise, you may not be able to achieve the desired volume. Also, especially with resistor passives, you need to make sure you have a match in input and output impedences, and you will need to keep the length of your IC's short. Otherwise, you may experience aberations at certain frequencies. They do not require AC power. They tend to be reletively inexpensive compared to active preamps at a given level of performance.

Personally, I like the transformer based designs (also known as TVC's) but I am not so hot on the resistor based designs. Most of the passive advocates will tell you they like the transparency and low noise of passives. Advocates of actives will claim passives result in a weak sound with no body, and will gladly sacrifice noise and transparency to get that body.
I haven't seen Sam's latest love affair? What piece is he hot on this time?

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?

Yes a lot has been written about the passive/active debate.

In a nutshell, the math (on which all electronics is based) goes against you when you run a passive system. The Lightspeed is no exception. You can improve the situation slightly by using setups that operate at lower impedances, but then your source has to support that operation, which is not something universal to say the least.

OTOH active line sections with marginal design will be easily beaten by a passive setup. So if you want to boil it down, on the cheap use passive, if you want the best you will have to go active.

How could a passive preamp improve the sound of the amplifier?

Anyone.....

Thanks to all have been responded. I have now begun my reschooling in basic electronics.

BTW, Brownsfan: Sam's current wunderlich (I know, bad pun) is the Music First Passive pre-amps. He wants to (again) do away with all active pre-amp because of all the "nasties" they ad to the music. Yes, but at what price and convenience??