Actually, that's not entirely correct.

Regardless of the OEM audio system -be Top HiFi (DSP) or HiFi- replacing the speakers on any BMW is not as simple as it seems, as the impedances of the OEM speakers are not the common 4 ohms as most of the aftermarket offerings. The OEM amps have not only the impedances of those speakers matched for power, but they also have internal crossovers matched to those impedances on each speaker.

Changing that impedance will change -not necessarily improve- the overall response of the system. In the BSW upgraded speaker combo for BMW's it seems that they actually took notice of the impedance of each OEM speaker and found a matching aftermarket replacement for them to offer this kit as a complete replacement with better response than OEM. Otherwise it will be hard for them to even say that there's going to be an improvement in sound quality while using the OEM amp.

There are ways to overcome the limitations of the OEM amp depending of the type of OEM system. If your X5 do not have any DSP processing then the OEM amp could be bypassed and an aftermarket amp with matching speakers could be installed. This is done by interfacing the aftermarket amp at the analog outputs of the tuner/inputs of the OEM amp. From there on the aftermarket amp(s) could take care of any speaker system that you wish to install. The only requirement for this bypass is that the aftermarket amp needs to be balanced input capable; all JL Audio amps, for example, are designed as differential input capable. If the amp is not balanced input capable then a differential passive or active LOC device (convert differential to non-differential signal) need to be used before the aftermarket amp.

If the amp is DSP then the OEM amp cannot be bypassed, as the whole tuner/amp communication is digital. As such, any aftermarket amp installed will have to be fed from the OEM amp analog outputs and not from its digital inputs. This is so because there is no aftermarket interface/converter yet that can be used to tap those digital signals at the OEM DSP amp inputs that then can be input into aftermarket amps and be processed. Replacing the tuner and amp is basically a non-issue, as this system is integrated into everything in the car (Assist, Telephone, PDC, Nav) that has to do with sound and alert tones thru the speakers.

Using the OEM DSP amp outputs as inputs for aftermarket amps is not as simple as installing Line Output Converters (LOC) and that's it. If the idea is to just add a sub, yes, it will work by using the OEM sub outputs from the OEM amp. However, if the idea is to replace all the speakers then it must be remembered that the OEM DSP amps have internal crossovers and EQ/tine correction processing. Therefore, to obtain a full bandwidth signal to be used as input to an aftermarket amp a device that can sum channels is required and the DSP processing (EQ) has to be reverted to flat.

All this could be done automatically by one of those new OEM integration processors like Cleansweep, 3SIXTY.2, Alpine Imprint or the upcoming JBL MS-8, by using one of those Zapco DC amps (built in DSP) or the professional -manual- way with an RTA and perhaps an Audiocontrol DQL-8.

Hope that this helps...