Well if the shop is giving a deal on the subs get them to give you a deal on an amp. I would go with just one amp and make it a mono 2 ohm stable. If you run the speaker in parralel it will give a 2 ohm load I beleive, if they are 4 ohm speakers. Maybe someone can make sure on that
Unless the voice coils are 6 ohm, or some multiple of that, they will not divide nicely into the 8/4/2/1 ohm catagory. We need to know if the voice coils are dual or singular, and what the impedance of each voice coil is. here is a calculator for figuring out total impedance in either a series or parallel configuration. http://www.the12volt.com/caraudio/boxcalcs.asp You should always use parallel wiring to string subs together, with the exception of dual voice coils which can be run in series on their own speaker. Granted, it won't damage the subs to string them all in series, but it makes for nasty sq along with an inefficient method for a car amp to develope power.
Sorry about that I huury and did the math in my head and it was wrong they are 4 ohm dual vc speakers had a pair before. I just thought 8 & 8 is 4 so another would be 2 but no it was actually 2.66667 sorry about the confusion