Rob and Beetle - I have also gone back and forth between wire configurations. My observations are similar to both of yours, the parallel configuration seems to add dimension and space. I hear more lucid harmonic detail such as obviously double strings on the mandolin or transient fingerings within electric guitar chordal work. And more lush and detailed (at the same time!) mid-bass. Such observations hold true blind.
I have also done some measuring. The only obvious difference is the presence of large, broad "bloating" of sub-sonics, perhaps 10+dB at 10 to 20 Hz with frequency sweeps beginning at 0 Hz, with accompanying group delay anomalies.
I am in conversation with Steven Hill of Straightwire as well as a knowledgeable physicist, plus my reading. Those expert opinions concur that whatever pleasant effect we are hearing, the technicals are more problematic. In other words: "don't do it".
One interesting trial was controlling the separation of the ++ and -- cables. Consistent separation is a technical requirement. Separating at 1.5" (my convenient wood blocks), served to lessen the "effect". Also spiraling the two cables lessens the effect. And, conversely, the effect is pronounced with widely separated runs (1' to 3' along the 10' run.)
I am getting some double helix wire from Straightwire where 4 conductors will carry the signal in star quad (opposite corner) configuration. This configuration is standard practice in studio cables, house wiring and industrial practice.
For my own listening I would choose the separated parallel conductors. Thank you Rob. However, my upgrade work requires science based solutions. I'm still working on the problem.
