With the OPPO HA-2, your talking about bypassing its ES9018K2M DAC, to use the A17's DAC.
I think people are discouraged at the prospect of under-driving any amp one might connect to the A17 Line Out.
With an output of only 0.245 V, the A17 Line Out is
eight times weaker than a DAC that puts out 2.0 V.
This effectively cuts the output of an external amp down to 1/8th the power it might deliver with a DAC that delivers 2.0V.
The A17's amp puts out 10 mW into 16-Ohm, but using the 0.245 V Line Out, the OPPO HA-2 might output only 1/8th of its rated maximum of 300 mW into 16-Ohm.
300 / 8 = 37.5 mW
Are you willing to pay $299 plus the cost of a Line Out cable to bump the headphone out from 10 mW to 37.5 mW?
Personally,and to each his own of course, I'd rather use the Sony
WMC-NWH10 cable + a USB A Male to USB Micro B male cable for getting digital out of the Sony A17 to the OPPO HA-2 - to use both its DAC and its amp and enjoy potentially 8 times the power, for 300 mW into 16-Ohms.
It would be clumsy, but I'm more interested in SQ than convenience or cosmetics:
http://www.head-fi.org/t/728401/sony-nwz-a10-series/4215#post_11519385
Another option is to drive an external amp with the A17's Headphone Out. If you really like the way it sounds currently (sans amp), you might find that double-amping with a neutral and transparent portable amp like the Meier QuickStep would give you a lot more bang for the buck (instead of using a DAC/amp).
Mike