I see, I see. So it's basically a transportable rig that's not great for the purpose that you wanted it to excel in; yea, that's understandable. I can see how you would prefer the DX90 to the DX50 + 6SMKII and that to the iPod + M8. Basically an upgrade in portability each step while not sacrificing sound quality at all.
Yes that's exactly the idea. At least this early I can tell you that DX90 alone is at least at the same level if not better SQ-wise compared to my DX50+RB+UHA-6SMKII.
Although not as powerful as the UHA-6SMKII, DX90 alone is good enough to drive my Beyerdynamic T1. I have a UHA-4 and the amp in DX90 is more powerful. I had a C5D and it could not drive my T1 to an acceptable level which is why I sold it. The UHA-4 will be going FS soon.
So if you look at it DX90, is much like a DX50 with a powerful amp built in. The improvement due to the DAC maybe subjective but the improvement in the amp section is undeniable and to me is well worth the upgrade to DX90 especially if like me, you are already tired or carrying a stack.