When you control H/K from the stock stereo, you can select spatial / festival / etc from the buttons. With an aftermarket HU, the mode button on the HKenabler does the same job - each press skips to the next mode (of course there's no display, but it's easy to recognize the sound style that you like!)
If you run subs from the H/K system, starting with an aftermarket HU is a great way to go - assuming it has sub outputs.
From the stock H/K, you have to pick up (rear) speaker levels and drive the sub from that. But you're getting "post-processed" sound, adjusted by the H/K amp.
With aftermarket HU, you can run subs from the RCA sub output and you have no H/K manipulation going on - and you can control it from HU independant of the H/K.
PS: Thanks, Mikey