ALTHOUGH, the WM8740 just sonically performs better via SPDIF than USB, this has been noted by MANY users, so if you CAN, you MIGHT enjoy the SQ of your E17 a bit better via SPDIF. However, using it via USB and letting it sip voltage and constantly recharge is VERY convenient. Oh, yeah, SPDIF sounds better even with 44.1kHz Redbook files.
Those numbers define the maximum voltage and current available from the various FiiO products.
If you need to kmow whether a particular FiiO product can drive your phones to a skull splitting 120 dB SPL, you also need either efficiency or sensitivity rating of your phones.