The S7 is not ideal as it has almost 6 ohms of output impedance, so the better source by far would be the Magni 2. And no, the Magni 2 is not too powerful, it has a low gain mode and you just need to be careful with the volume pot.
Try testing between the Magni 2 and the S7, theoretically the S7's output should be brighter, as in the lower frequencies the IM02's impedance drops down to as low as 16 ohms. It shouldn't be that bad though, between my iPhone 6S (~4 ohm) and the 7's dongle (<1ohm), the dongle just had a little more bass extension.
Now if it had >10 ohm output impedance, thats when I would say to throw away that shitty source