Actually, I'd suggest considering the HKenabler as a first step (unless you've already ordered speakers etc).
I know I might be biased, since I created it, but you will find that the aftermarket HU audio features do translate quite nicely through the H/K amp with the HKenabler. And you will be able to push the system a little harder by careful adjustment of the volume levels.
Plus, if you then decide it's not good enough, you can re-use the harness adaptor part and just ignore the HKenabler module itself.
Starting from the H/K actually makes your life MUCH easier since (a) the speaker wiring is upgraded from stock and (b) it's all starting from the H/K amp position which is easy to wire aftermarket to.
The only challenges are running the RCAs from front to rear, and eliminating engine whine!
My GBMINI has Boston Pro speakers in the front, and JL audio 6" subs in the back, all driven from a Rockford Fosgate Punch amp (the H/K link I posted above is someone elses car) - when I switch off the subs, the Boston Pros are no match at all for getting the low frequencies ... I hope you find the TS-D901Ps better able to substitute for a sub
