@dixlon
You can feed your audio through the HD+ and back into the 990, but I chose to handle A+V separately, 990 gets no video at all; I can switch inputs through my Pronto remote by switching the audio for the 990 and the video for the scaler.
Hope this helps?

As for the 990 sound quality:
I looked at the frequency response for the unit, and it's straight as a ruler.
Although there's more to sound quality than just the response (macro- and microdynamics, noise & hum) IMHO the 990 is the best example of "the straight wire" version of any receiver or preamp I heard thus far.
I connected my CDP to the CD input and switched over to direct feed into my Arcam P7 final amp (through an ALPS potentiometer) and I could not hear any difference, with no 990 "bypass" setting applied.
And I listened to some big Mahler symphonies, plus a Cecilia Bartoli disc, and dynamics seemed fine.
Now it's possible that some people prefer a colored sound of some sort, warmer, more analytical or with the pronounced 2nd harmonics of the "tuby" sound, but IMO a pre should be as linear as possible, with an accessible tone control (sorry for the blasphemy!).
What I would like to know is in what respect the AVM units sound better, and I certainly don't mean this as a criticism!
All said, there are brands with a specific sound, I am pretty sure of the "Arcam sound", can't lay my hand on it, but it's there!