While I bought this thing as a Bluetooth to S/PDIF (COAX) converter, since I have it I figured I might as well actually give it a listen as both a Bluetooth and USB DAC as well.
As USB DACs (iONE vs. Nano iDSD Black Label):
The firmware version affects both audible and measured performance with USB input here, so pay attention to that.
Broadly speaking it’s not very far from the iFi Nano iDSD Black Label when fed over USB with output into an external amplifier using the iONE’s RCA outputs and the iDSD’s 3.5mm Line Out. When used purely as DACs, they have a very similar signature and technicalities and it is really only the presence of some low-level edginess/hash (which was firmware-dependent) with the iONE that puts it behind the iDSD.
That edginess/hash was NOT present using firmware 5.2 nor 5.3C. Also, if I applied digital volume control ahead of feeding the signal to the iONE, and reduced the output level to a maximum of -1 dBFS (normally this would be 0 dBFS), then this edginess goes away. Unless you need USB -> S/PDIF conversion here, this is a non-issue.
I suspect any differences in final sound, on the 5.3C firmware, are down to the output stages of the units rather than anything else… But the iDSD Black Label is definitely ahead here. We’re not talking major differences. Maybe 1 or 2% at most. But if you don’t need/want the Bluetooth functionality, the iDSD is the better sounding unit, with the bonus that it includes a very nice headphone output and IEMatch for IEMs, as well as a built in battery for use on the go.
As a Bluetooth DAC:
The most obvious differences between using the iONE with a lossless USB source vs. via Bluetooth (both using an identical 16/44.1 FLAC source) are that the Bluetooth connection loses some detail, exhibits a slight bass emphasis (more in terms of a little bloom than in level), and that there is some softening of transients.
It is an inoffensive difference, and isn’t plagued by harshness or grain. It’s just a bit smoothed off vs. the USB input.
AAC vs. aptX
This is a fairly easy one; AAC sounds marginally better than aptX here. Both are notably better than SBC, but AAC just takes the cake. If your source device lets you conveniently choose your protocol, then I would go for AAC personally, but the difference is not as big as the change going from USB to Bluetooth in the first place.