Agree that the Master 7 is in a much higher league but the comparison is hardly fair, price wise. We're talking about a >2k usd device so I'd hope it does things better. For the dac/amp question: tough but if you want to try tubes I'd go for a used tube amp first, the precision of the dac becomes less relevant then, all things considered. And it'd be a hell of a lot cheaper, really going to a significantly better dac will cost a lot more cheddar than improving or changing the amp part. Not to mention that amps depreciate a lot slower than dacs.
I find head fi so frustrating. The only thing that we need to be concerned with now is the transducer . Spend your money on music and headphones not dacs/ amps. If the odac/O2 can give the volume you need then you can move on from the amp/dac conundrum! I put money on the fact that the hd800 will sound the same on the O2 /odac as any transparent TOTL dac/amp in 1000( insert any year) years time. The transducer is the sound!!!!
No doubt. One of my friends has a whole bunch of amps and DACs from his trips over from Japan and Hong Kong, and the O2 and ODAC certainly get overshadowed by some of the larger units. He buys stuff used though, so his things are often 1/3-1/2 the price.