I opinions will always vary to a huge degree when it comes to high end audio equipment. I am finding that a lot of amps and DACS out there aren't necessarily better than the other just different. Some however, are far better for the money and that is where quality research and testing really comes in handy.
I've heard amazing things about that OPPO HA-1. It's definitely major bang for your dollar. I would take it over the HDV800 anyday and I'd save around 600$!!
Actually between my system's set up for headphone use I think I spent around 200$ more than you did, and you claim that the DAC portion in the OPPO measures up to the ARCAM irDAC? That's amazing then. The ARCAM performs like a 1500$ DAC and to my ears it's not a 'mid tier' DAC. It's completely high end, so I guess you are sitting pretty with that OPPO. Sounds great. Love to try it some day. Well done and enjoy.
Lastly I'm sure the amp portion in the OPPO is every bit as good as say, the Burson soloist. I have found that amps don't make as dramatic a change compared to adding DACs into your sound chain. Getting a good DAC is far more noticeable I have found.