My experience has been that thicker cables may degrade the quality. Degradation is minor, so - it's just lesser evil that we chose. Cable inductive resistance attenuates higher frequencies. It can be mitigated by using SPC that compensates frequency dependent loss, putting skin-effect vs inductive resistance effect. Success of that compensation depends on carefully chosen cable makeup(# of SPC/copper #threads/cores, braid). If cable is super-high quality is designed to be neutral and not change highly precise original tuning. Or, (like Beyer/Senn) -
IEMs are tuned for thinner, more comfortable cable. (Love German companies pragmatism). Has anyone tried to replace ier-z1r or M9 cable? Sony stock cable is the only one that sounds correctly with those IEMs. Daa, they're tuned for that cable.
Many audiophile companies however, lately started to get on the bandwagon, and include substantial looking and feeling thick cables to satisfy the expectation of expensive luxury product.
Xelento has very thin cable for comfort reason and IEM is subsequently tuned for that cable. If you attach thick SPC - you will change original tuning. I've done that with Xelento because I wanted brighter sound. Accuracy wise it was inferior, just sounded better with some genres. When I used EQ - I've gotten better results with stock cable. Same with Orb.
With speakers it's not the case, because on that scale cables are usually not overly thick. With speakers we have several orders of magnitude
higher current, and as a result of that - inductive loss is much higher. On top of that higher frequencies are lost through capacitance of the
longer cable. So, with thicker multi-threaded cable the loss is compensated. (Amount of surface concentrated in the same diameter matters).
Let's say, If IEM puts out 100db with 10mW of power and average speaker needs let's say 10W(probably more). Take 5mm thick 8 core cable and make it x1000.
It will be thicker than fire-hose. It's already too much for IEM (IMHO), not too much for a loud-speaker.
So, what we (audiophiles) are doing - is just fine-tuning IEM to our preferences without the trouble of EQ-ing.
But, when cable is too thick - it has higher capacitance, that puts more load on power Amp, (often unpredictable, how that output handles that kind of load). Both capacitance and inductance of the cable may also react unpredictably with IEMs own. Lower conductivity doesn't benefit IEM.
So, my philosophy is - to minimize thickness when possible.
I think that thin 8 core braided cables should be a reasonable limit, that isn't pushing the tolerance of the Amp output too much. So many times I've connected thick cable to correct minor FR issue and found that sound is not clean anymore. Sometimes it boosts also frequencies you don't want to boost (happened with ORB), and with some sources you also get more distortions.
If only EQ on Android was not such a pain in the neck...