I used a Genesis Profile 4 (UK amp) but there are lots to choose from.

I wanted "musicallity" with good midrange clarity, rather than splashy cymbals and thumping bass (which you won't get with standard speakers anyway) the reason for now wanting a SUB (and SUB AMP)is to take the low frequency duties away from the Genesis AMP and speakers to allow the power of that AMP to be concentrated in the mid range, rather than being squandered on the low frequencies that the standard speakers can't reproduce anyway. The extra low frequency power will warm the sound up and I hope reduce the slightly "woody" low mids which I think come from asking the speakers in the door enclosures to reproduce lots of low frequency sounds which is causing this resonance. (Just a hunch!) Price is the only thing holding me back. Dealer wants £1100 for a custom job. 10 inch diamond with Genesis SUB amp. JL Audio are bringing out a stealth box for the X5 shortly, and BSW have one already, but the shipping costs to the UK kill it for me.

The OEM amp is in the boot (trunk!) I have been told that one of the problems with the OEM AMP (or at least the UK version,don't know about US) is that it contains active crossovers and I think these need to be substituted with something else to get the best out of the system. Mine was fitted by local specialist who has spent a lot of time trying out different options for the X5 but I don't know exactly what they did iro the crossovers (probably just a good sales pitch!)

Hope this helps,

Oscar