The Reference series, while great drivers, really need a bit more power than the stock amplifiers in the X5's can produce. If you aren't sure whether or not you want to upgrade the amplifier down the road, my recommendation would be to stick with the Discus series -- they'll sound better on the stock amp, and they also respond extremely well to an amplifier upgrade. It's a win-win!
Note that we will have an amplifier upgrade package priced around $470 coming out very soon!