Does Harman Amp fit standard E53?

I know its cheaper to go aftermarket, but here is my situation: I have changed the speakers all to Alpine and used to have them amped. I don't want aftermarket amps on show as I may be selling/part x-ing in the next year or so, so now have them running off the stock amp. I'd like a little more power and can source an HK for a reasonable price. Does anyone know if it would be a direct replacement, which would be excellent, or if not, how I would replace it?

Many thanks.

I 'm not familiar with the HK system on the X5 but most BMWs have crossovers in the amps, customized to the OEM speakers, which makes it pretty much impossible to swap out just the speakers and still have optimim sound quality.

The HK amp is not a plug and play amp for the standard sound system.