I switch between A4 and HQPall the time and would hate to have to choose to only have one of them. Just talking straight PCM, both upsampling (using the best filter for each album on HQP; none of the filters ever sounds the same as A4) and playing thru my cheapish system (my speakers have plenty of grunt but diamond tweeters that can be a touch hard, which makes Audirvana painful to listen to and definitely gives an advantage to A4 which is more rolled off).
I think there is a pattern to my choices. I see that, when I want to turn up the wick and put myself inside the soundstage a bit, I go for A4. The sounds merge more into a coherent deep stage. The thing A4 does really well is give you the physical force of the music: someone blowing hard against a reed, the tension of a drumhead or steel strings. You can really hear the stick on the drumhead. It does a great job on cymbals, too: they aren't white noise, you can really get a feel for the size of the different cymbals. The bottom end has that force: someone dropping bombs on the kick really gives you a start. Vocals are right in your face and super believable. A complicated multimic mix, however, can be too much for A4, and you quickly get a list in your head of the albums you shouldn't feed it. I think everthing I just said would come under the heading of 'analog', I could write the same of an SET amp.
Switching from A4 to HQP, the first reaction is like the whole sound took a shower: so much space and clarity to each instrument. Suddenly you can pick apart the layers of Ellington's horn writing, follow the logic of what Ron Carter or John Paul Jones are doing. The big coherent enveloping soundstage and the sense of urgency are gone; there are just instruments picked out in a very black space, and I find that I use HQP when I am going to play things at lower levels rather than take a bath in the sound. With the bloom coming off, you can follow the interplay of a drummers hands and feet, separate out the original sound and the effects, basically it tells you what's there and what people are thinking when they play. HQP handles any kind of mix, it's not phased by density and the bottom end is a good weight rather than being punchy.