Sorry to revive old thread, but was just experimenting on my sr6007. To recap the main point of the thread, AcuDefTechGuy called Denon who told him this:
I finally talked to someone at Denon regarding Pure Direct & Direct 2.1.
Pure Direct 2.0/ Direct 2.0 is simply using the DAC to output 20Hz-20kHz signal.
The only extra step Denon has taken w/ PD2.1/D2.1 is using a discrete analog crossover circuit to output 20-80Hz signal.
This discrete analog crossover circuit is not DSP. It is in ANALOG form. It does not need to be converted back to digital (ADC).
So the answer is Denon uses one extra step with a discrete analog crossover circuit to simply output the bass signal to the subwoofer.
Apparently this extra step requires extra money. Onkyo, Yamaha, and most people don't want to implement this extra dedicated circuit.
So Denon claim there is an analogue 80hz subwoofer crossover in some Denon models to allow subwoofer play in Direct/Pure Direct Mode, although the manual says nothing about this. I wanted to test this claim so I put my SR6007 (same as a Denon 2313 or 3313) on pure direct mode and played around with a frequency airplay app to see what the bass management was doing at different frequencies.
Seems that by adjusting the crossover on the main speakers (set to small) while in Direct Mode I'm able to change the amount of bass being sent to the sub. This shouldn't be possible if there's an analogue crossover set at 80hz. So, on the SR6007 at least, I think there is a digital signal being sent for bass management processing.
Either way, there is a noticeable difference between Stereo and Pure Direct. I don't know why or how, but the centre image seems much stronger in Direct Mode. I had to check that my centre speaker wasn't playing! Stereo mode sounds like 2 separate speakers playing. I've heard others say the same thing and I was skeptical because there's no logical reason for it, but there it is.