Best way to A/B two different streamers

Easy. Buy a Wiim Pro.

Connect via coax. Compare. 🎯

A bi-directional switch is a cheap investment for a quick A/B test. If you’re not concerned or counting on that component affecting the sound in anyway then go for it! You can level match your PC’s output to that of the streamer (pre dac) and compare away.

I wouldn't use a switch for several reasons:

1. I keep noticing that the differences between sources and between cables tend to be quite subtle sometimes, albeit still very important. Quick A-B-ing is not the best way to reveal them IME.

2. The logic behind using an A-B switch is something like this: if A>B (A and B are the components to be compared, > means "sound better to me than") then A+X>B+X is also true (where X is the sound impact of the switch and additional cables). This logic is flawed IMO. Just a crude example: say A and B sound very much alike, but A has better treble extension making it preferable. If switch X is rolling off the treble a bit itself, the advantage of A will not be heard anymore through the switch. Another example: A is a bit more refined and detailed, but also a bit boomy and PRAT is lacking compared to B, maybe because the bass of A is more extended and is exciting some room modes more, making it woolly and swallowing dynamics - so I'd definitely prefer B. The switch could reduce and tighten the bass a bit so that A+X (switch) sound more balanced and dynamic, while B+X sounds a bit bass light, thus making me make the wrong choice (A). Of course these are crude and oversimplified examples, but you get the idea.

I think your best bet is to listen to a song through A, than repeat the first 10-12 seconds, then listen to the same song through B, also repeat the first 10-20 seconds and finally listen again through A. I find the A-B-A testing much more revealing than the simpler A-B. This way you will quickly notice the main differences between the sound of A and the sound of B and then confirm and refine your findings through subsequent auditions (other, as different as possible, tracks to listen according to the A-B-A - or B-A-B sometimes, for a change - protocol).

I would not use a switch. Will confound the analysis.

After both are broken in I would try one day one then one day another. Do that a couple times to see if there is some really huge difference. Then I would listen to the music from one for a couple weeks. Then switch, listening for a couple weeks. Do this a couple times and the differences at all levels should become obvious. 
 

If you really want to understand the differences at all levels you have to listen to the music and not the equipment and the differences will unfold.

@ghdprentice - I tend to lean towards this approach.

OP - You should have a sonic memory for the computer, so listen to the new streamer for a few days or weeks and see if you feel that something’s different.  Then go back and verify. If you can’t form an opinion then you’re splitting a hair not worth splitting.

I have found that I have formed an opinion with something new and then when I switched back it was clear that my sonic memory was flawed.  Other times I’ve found it a relief to switch back. Other times the difference was so immediately obvious that I never bothered to go back.