The Human Brain hard at work! Pre-conceptions essentially ruin any chance of a fair A/B test in most "perception" type scenarios. That is what we get for being "Human".
FWIW - The Zeros and Ones in the SPDIF stream have NO CORRELATION to "Warm" or "Tighter Bass" - they have no correlation to audio AT ALL. You can not look at a SPDIF stream and say "that stream is a low frequency - I can tell" - the 1's and 0's are not "Analogous" to analog audio (that's why it is called digital, and not analog
). 1's and 0's do not make audio - the DAC and its reconstruction + filtering DO. The only issue that can be direclty applied to the resultant audio is jitter (still not a direct relation to "Analog" audio), and dropout. Lose the DATA, and you loose the associated Audio. Errors do affect sound, but I doubt swapping from a working 6' optical to a 6' glass fiber cable is reducing errors (unless the old cable was flawed). Just sayin'...
Humans are one-of-a-kind! What can we do? Just enjoy the multiple imagined realities we as humans create. I don't believe ANY more A/B tests unless they are true double blinds (don't see many of those in the Audiophile world, or those $1000 AC power cords would be history by now
).
Good converters can make a world of difference, as this is where the "work" is done, but as long as the DAC is getting a fairly jitter-free clock, and an error-free data stream, digital interconnects will not affect "sound" (its DATA at this stage, not "Audio").