Do whatever you want, if you don't mind your signal going though an additional dozen op-amps and who knows what cap's before it get's to your 789, more power to you. Listen to them and make up your own mind.
@ Phoniac, im curious what you mean by the 789 not being worth it behind the DAC. Are you implying that the built in head-amp is equal to or greater than the 789. If so, how did you come to that conclusion? This is an honest question, I want to be clear that im not being sarcastic, as Im considering pairing these two together myself. If you have experience directly with the two, I'd love to hear it. My main phone right now is HD650.
As far as my experience, I still don't have as much head-time as i'd like, with a broken in unit. But I have the ADI-2 DAC paired with the Schiit Jotunheim. I have been trying out the built in head-amp, and the Jotunheim. At first I felt the Jotunheim was clearly superior, but i've been burning in the head-amp, and doing more critical listening. Now im not so sure, it seems more a preference of flavor then technical ability. While the Jotunheim is technically capable of pumping more power into a 300 ohm load, neither device is wanting for power/volume.
I find the bass notes clearer, and the high's also clearer on the Built in amp. Though the Jotunhiem tends to be more "3D", and tends to be superior with regards to image, and placement of instruments. It's bass also hits a bit harder, but is not cleaner.
Jotunheim is superior with regards to a sense of "Air" and feeling the room. Perhaps slightly more "natural". It's not cut and dry though, as the ADI's built in amp does have superior resolution across the board, with regards to bass and high's. Neither are sibilant or harsh with the ADI's dac driving things.
I find Jotunheim more musical, and ADI leaving me wanting, for something.. I do want to pair it with a higher end amp. Im considering, THX789, Schiit Valhalla 2, Monoprice Liquid Platinum, Nerochrome HPA-1, LYR3, or perhaps Feliks.
A quick comparison to my previous DAC, Schiit Bimby GEN5:
Find the RME to be far more detailed, with more resolution, and a much cleaner presentation overall. The various feature's of the unit are a huge bonus, particularly the PEQ, crossfeed, volume control, multiple out's, etc.
Bimby simply "falls apart" at certain types of content, this is true for Mimby and Multibit Dac module as well. All three are more "Natural" sounding then RME ADI-2. Though, that said, I find RME-ADI-2 to be warm-neutral, and quite pleasant to listen to, I would not call it Sabre clinical. Certain passages of music cause Bimby/MImby/Multibit Module to fail, spectacularly, the RME-ADI-2 has none of this, for awhile I thought it was Jotunheim, but it's in fact the lower end schiit multibit dac's. A great example of this behavior is in the track "Everyone's looking for home" by Sam Outlaw. At about 2 minutes in, there's a huge horn crecendo, all of the lower end schiit multibit just sounds awful in this passage. To be fair, the bimby sounds far better then the Multibit Module, but it's not close to the RME-ADI-2 here. I found this "issue" with these lower end multibit options particularly irritating. I don't know if the problem persist with Gungnir and/or Yggdrasil.
According to RME the ADI-2 DAC needs no break in. I don't agree based on what I've heard with the unit. Sounds kind of plastic at first, it takes 100 hours or so before it open's up. (particularly, the head-amp)