Wiring preamp question

So, I'm working on a traveler guitar concept, and what I want to do is have an onboard preamp so that I can plug into headphones or have a small speaker to plug into. I have found http://www.guitarfetish.com/Onboard-25-Watt-Guitar-Amp-Drive-a-speaker-OR-Headphones_p_567.html this which looks good to me, the question is how to wire it.
My thought was to have a master volume and tone with the guitar (which has 2 humbuckers already). My question was, does this go in as the master volume? or as a seperate pot. The other thing mentioned in the description of the product is using a switch to turn the amp on or off, would this be needed or recommended? To install a push pull switch would that then be the master volume with the preamp pot being the preamp volume and would it be needed to function? Instead could I have a simple switch to turn the preamp on or off, and would it function properly through a standard amp then too? or is it worth to shell out the only $9 more for the push pull? I'm only really asking because I am low on space to keep the size down in this guitar.
Could anyone make a simple diagram of how this would layout for me? I'm not much used to electronics yet.
Any help is greatly appreciated, thank you very much.

For on/off a stereo jack is usually used w the second contact used to ground the battery negative like an EMG set up. Pluging in turns it on. Usually I don't try to disuade/redirect the idea in the post...but a headphone amp is tiny and will have effects. For that matter my Zoom box that does everything but walk the dog has a headphone out and it is no hassle to travel with its 10x5x2 inches and less than a lb. in weight. Unless you want to have a low Z out in general, an on board pre/headphones is unnecesssary.

Well, I was considering putting in a speaker as well, but I was assessing my options before going through with it. The other thing I was thinking was building a Noisy Cricket amp http://www.beavisaudio.com/projects/NoisyCricket/ but I wanted to know my options and like the idea of plug and play better than having to deal with a separate box to carry around with it. This is a dedicated travel guitar, so I'd like to keep it simple but self contained.