Some examples of what my 8.4 controls in my 2500:
All steering wheel controls
Cargo and backup cam
Driver/Passenger heated/cooled seats
Heated steering wheel
Mirror dimmer controls
Sirius XM Travel Link/weather/fuel pricing
Onboard hotspot
Front and Rear parking sensor controls
TPMS
Exterior and Interior vehicle lighting controls
Door lock controls/behavior
Auto-On comfort controls for HVAC/seats/steering wheel
Engine off safety/light controls
Trailer brake settings
There is no other HU on the market that will integrate with all of these controls without a bunch of adapters, harness work, hacking or some other BCM modifications through Alpha OBD. Any of which that splice into the harness, eliminates the factory warranty and if you screw it up? You're completely screwed, requiring the purchase of a new ECM/BCM or both for thousands of dollars. The closest option might be the Tesla style 10.4" Android's that can be retrofitted but even those don't work fully without a BUNCH of work, harness mods and a high price tag. Most people who have retrofitted did it purely for the screen and still don't have 100% integration yet. I would even venture to guess that even those come with Bass roll off so it would be a worthless upgrade for a "SQ/Audiophile" build.
I'll reiterate, the PAC Amp Pro or AudioControl LCi or the DSR1 (moreso lately) are reliable and proven for this specific application. I prefer PAC for the harness integration. (I'm unsure if the DSR1 has this)
To the OP: I don't think anyone here as specific experience comparing the DSR1 vs an AP4 but it appears that the DSR1 will get the job done but the AP4 provides you with the option of choosing your own DSP (if you want) and provides more flexibility for you in the long run for this specific application.