1. What purpose does pre-amp serve other than allowing the user to control the volume?
Nothing else really, although volume control is just the simplest way to put it. For the particular details on how it does so, here's a brief article on Wikipedia that came up on Google: http://en.wikipedia.org/wiki/Preamplifier
2. Does the pre-amp effect the sound quality?
That depends. One reason why the passive preamp was designed was specifically to avoid active components that add their own sound. To some, altering the sound is a plus when for example the circuit is designed to color the signal with tubes. So for example a hybrid amplifier isn't just a single box integrated amp - a tube preamp mated to a solid state amplifier essentially works the same way.
In any case an active preamp can be designed to be transparent; the problem there is that it usually takes a lot of work. A passive preamp isn't any less complex though - you could end up with signal losses for example. Personally, if all you need is just a preamp (and you already have a power amp anyway), might as well use a good quality passive pre; if you need a single box solution that integrates a headphone amp, maybe a DAC too, no need to sweat the details (well, maybe get a solid state to be sure, but I'm guessing you like tubes since you have a Woo).
3. Does the pre-amp effect the output power of the power amp when powering the speakers?
No, it just affects how the signal interacts with the power amp downstream. The final output volume is then determined by the preamp gain, the power of the amp, and the efficiency of the speakers.
4. Hypothetically if I had two WA2 wired together in a daisy chain, what effect would that have on the sound output?
What for? You'll just spend a lot of money in order to mess up the gain structure. Regardless of how many preamps are in the chain your signal isn't going to get stronger, or not without distorting at some point (maybe even before it gets to the power amp).
That said in some cases there
are more than one preamp (or a preamp and a simpler gain control on the amp) in some systems. Take for example active monitors - each cabinet has its own amp and gain control, and then you use a preamp to put a single control within arm's reach. In car audio systems, there is a gain control on the amplifier in order to match the gain in the final output. For example, in simple systems, at the very least you'll need to have some control on the subwoofer amp gain so it doesn't overpower the front speakers, with a single knob to as master volume on the receiver. In the most complex systems, since the DSP needs to apply time alignment to each tweeter, each midrange, each midbass, and each subwoofer, and it's not like you can always get a 5w/25w/75w/250w amp with the right gain (given the highly variable power handling and efficiency specs, plus acoustic properties of the cabin and installation), you'd need not only the amp and the master control on the receiver, but the processor can also introduce further reductions on for example the tweeter outputs. For those who like blowing their windshields off at the parking lot, you can increase the signal voltage from the source, and car audio amps are designed to handle that much (some Alpines used to have 4v; some Pioneers had 6.5v; some Eclipse had 8v).
