I trust Lee opinion on uha-6s mkii. He had both the O2 and the uha-6s mkii and he prefers the uha-6s mkii over the O2. It should be said that he did not try the ODAC with the O2.
I urge anyone who can to try ER4S+O2+ODAC, truly an exotic sound. The unit after 40 hours of burning in has given the Er4s the lover bass extension bass. It does not boost the bass but gives it more refinement and detail. The top end detail was great from the day one without the burn in. The sound is definitely not V-shaped but for some reason the mid-range feels less prominent than the upper and lower range.
But this combination is very transparent. I have heard so many things that I was not able to pick up with my other amp and dac combo(faint back ground vocal, singers breathing pattern, can be distracting at time but it is very fun!)
I also enjoy the HFI 780+O2+ODAC for Jazz greatly. They do a decent job on electronic.
I will try to use the ODAC+O2 with the XBA-4 and k702, fidelio M1 and SHE9900. I might buy the HD 598 and try it out but for now in summary I finally found a combination for my beloved Er4s. I can stop the search and be very happy with this combination!