I'm sorry-I read this post when you originally made it and meant to respond, but forgot after work.
Here's my recommendation first of all. Go with multiples of 4 when it comes to setting up the speakers. The reason has to do with ohm loads and that good garbage. Six is an odd number that CAN easily work, but I think 8 would be a better option. I'm sure you can find a deal on speakers on eBay or
http://www.bsless.com. I have a couple of friends who have bought from the no B.S. site with good luck. Ebay will likely be a little cheaper.
If I were you I'd keep that amp on the sub. It's well suited to that purpose, but will not run 4, 6, or 8 speakers real well. Go and get yourself a 4 channel amp. Remember, used is ok most of the time with name brand stuff. Make sure it is capable of running 2 ohms stereo if you want to run 8 speakers. Again, name brand stuff should be able to. If it gives a "bridged" rating or a power rating of 2x ...watts@4 ohms it will work fine if it's a 4 channel amp.
When you go to wire in the speakers you will be wiring them in pairs. You will have a right front pair, left front pair, right rear pair, and left rear pair. Simple, right? Run the positive and negatives from each speaker to their respective positive and negative outputs on the amp. Alternatively you could use heavier gauge wiring from the amp to the first speaker and tie the positive and negative terminal in to that wiring from the second speaker in at the first speaker of the pair to save on some wiring.
You will also need to get a head unit with 3 pairs of RCA preouts. You could use high level inputs, but RCA's are WAY easier and cheaper in my experience. You will run the left and right front and rear outputs from the headunit to the head unit to the amplifier. You could use the third output (marked sub) for running your subwoofer or just not use it for now.
The end result will be that you will still have stereo sound AND a working front and rear fader. That way you could listen to just the 4 speakers in the back or just the 4 speakers in front which is nice if they are in different rooms within the bus. The amp will be loaded to 2 ohms stereo which is a pretty easy load for it to handle. It will get warm, but in my experience a 4 ohm mono load on a 4 ohm stereo amp (a 4 channel is essentially two of that setup in one) is tougher than 2 ohms stereo for an amp and most can handle that JUST fine.
Now it's time for my word of caution on speaker and amplifier selection. Get decent speakers. I know you're buying 4 pairs (I hope I've convinced you of that) so it can get spendy, but look for the ones with the largest frequency response range and the greatest efficiency noted in dB per 1 watt meter. The difference between a speaker that runs 90dB w/m and one that runs 93db w/m will be twice the sound for a given input power. That's a pretty staggering difference. :shock: I would look for speakers in the 6.25 or 6.5 inch range. They actually have a larger motor structure on them than most 6x9's which have a 5.25 or 5.5 inch speaker's motor structure with a large, odd shaped cone. Remember, soundwaves travel in a circular pattern so if a non-circular cone is pushing them the waves will hit the surround at different times and it won't sound AS clear. This is why I'm not a huge fan of the odd shaped subwoofers out there. They might move more air for a given footprint, but they don't sound as tight. I guess it's just a personal preference....
When you go looking for an amplifier you are going to need a 4 channel amp capable of running 2 ohms stereo as I posted before. Having a level adjustment and high pass crossover is important in my book, but a lot of that can be accomplished through a decent head unit. So what's the most important thing I look for? Look for the nominal/continuous/RMS wattage rating. Peak or max power mean NOTHING. There is no god standard for figuring it out. I question whether some amplifiers would be capable of putting out their peak power when struck by lightning. Match the stereo 2 ohms RMS output to the speaker's RMS input and you will be happy. Also try and stick to a THD (total harmonic distortion) of <.5 as you won't be able to hear it at that level. If the THD for a given output is 10% like some amps have it will sound like crap when you turn it up.
Lastly, make sure you use quality wiring for everything. I'm not going to say you need to run Monster cable. Heck, I think the stuff is a bit of a rip of, but run quality stuff. More strands of wire in a given gauge are good. Size the wire appropriately for the run you have to make and make sure you have a good ground. Lots of people end up putting a capacitor in their car to fix dimming headlights when all they really need is a 4 gauge grounding strap from the battery to the body. Good wiring DOES make a difference.
If you have any more questions or need a drawing I'd be more than happy to lay something out for you.
