bits is bits

I replaced the ethernet cable between my streamer and switch with SFP optical fibre. As far as I can discern, it made absolutely zero difference to sound quality.

That's in a highly resolving system. Both the Linn Klimax DSM/3 streamer and Melco S100 switch have SFP ports and I used the often recommended Finisar optical modules. 

Other changes I have made in the past like adding the Melco switch and upgrading the streamer have been beneficial, but the SFP optical fibre wasn't. Others using the same streamer have reported improvement from changing to fibre. Maybe the Melco is already providing sufficient isolation.

I have both Qobuz and Tidal-have for years. I wouldn't try to say what sounds universally "best" to someone else- anybody that does that can't possibly know-people have different, individual preferences-I know people who say they prefer Tidal, Qobuz, or Amazon, for that matter and blind tests show that preferences differ. That's no shock, just common sense. 

Personally, I find very little difference. I prefer the Qobuz interface, so use it more but I've heard tracks I thought were best on Tidal. That said, I doubt I could tell the difference in a properly conducted blind test. There is also the issue that any kind of SQ test between music services is useless unless you know you have the exact same versions on each service-the same master, etc. Likely why some tracks sound better on one service-some on another.

I suggest anyone choosing between lossless services pay a lot of attention to which interface you like best-that might dictate which service will give you a better experience and that also is subject to individual preference. If you think there is a significant difference in sound quality-listen for yourself. There is no universal "best" only best for you. 

There's a lot to "unpack" with digital audio. In a nutshell

#1 you can't assume Tidal and Qobuz received the same file in the first place

#2 assume the same TCP/IP packets all arrive to your streamer bit perfect, now look at what happens next from the renderer, DAC, and eventual output.

Same bits, different results

Nope, we can't assume TCP/IP packets all arrive at your streamer bit perfect, they arrive at your modem bit perfect. One's network does affect the encoding of these bits.

We all assume a lot. We assume that all streaming services receive the same source file. That might not be the case. We also forget that until that "music" passes the DAC, it is in fact not music, but a digital file. Any software before the DAC can, and will affect the sound. Like Spotify for instance, has a bass boost over all other services, like a "loudness" that you can't turn off. In the car it's fine, but on my home stereo, it's too much. 

We also assume that all streaming services, use the same software/storage/server/OS etc. They are all different ways to deliver the bits to your DAC. Think all things contribute to slightly different sonic profiles.