[GBMINI]

Mikey,
I am still working (occassionally) on a permanent "gadget" to run the HK amp (at the moment I still have the guts of a stock HU tucked in the trunk).
The problem is that the HK amp does the volume/bass/treble/balance/fade stuff (as well as the MODE); obviously you can set the HK amp to flat bass/treble/balance - but you lose the fade control ... the biggest issue is the volume. Theoretically you just tell the HK amp to run at max volume, and vary the speaker levels out of the aftermarket amp (which does work) - the problem with this is that at full volume, the HK amp generates noticeable audible noise, which you hear when the aftermarket HU is *not* running at maximum volume.
If you set the HK amp to a lower volume (so that the noise is inaudible) then the maximum you can ever get out of the system is not as loud as people might want.
Finally, replacing the HU will not give you (much) more overall volume, since the speaker levels into the HK amp are limited to somewhere between 3-5V (not bothered measuring exactly what it is) ...