I use the Ibasso DC-03, with a PC and like it, I lost mine last week, but just ordered another one.
Are you going to use the DC-03 with a computer or plugging it into your phone? (or both or other).
So I'm assuming the DC-04 is decent.
Small speakers will not be able to provide the quality of sound, compared to good headphones.
I usually check out Monoprice for stuff, good bang for the buck.
Most receivers, to save in manufacturing costs, use the same amplifier(s) that drive the speakers, to also drive the headphones.
So it's a little crude and not great for lower impedance (Ohm) headphones.
So headphones that are 250-Ohm or higher are what you want to use for most receivers (or...
That should work.
Would assume it would be cheaper manufacturer the X7's RCA input to automatically convert all incoming audio to digital.
Instead of adding an extra analog audio route (RCA to headphone jack).
But I'm just guessing.
RCA (analog audio signal) input on the Edifers can easily pass a 24-bit/192khz signal.
Any analog audio signal from an external DAC should work fine with the Edifer's RCA (red/white) input.
But I think I would just stick to the optical input.