I agree with Atmasphere that bypassing a coupling cap results in smearing and a generally incoherent sound. This is especially true when the coupling cap is a high quality cap like a V-Cap CuTF (my favorite). And this is also true even when the manufacturer used bypass caps in the original design. For example, for a long time ARC used a cocktail consisting of a large value film cap (either a Rel-Cap or MIT) bypassed with one or even two .01uf InfiniCaps. You would think an established company like ARC would know what they were doing but not in this case. For each of my ARC preamps, I was able to improve the sound quality by removing the bypass cap and letting the base cap (especially the 5uf MIT base caps) do all the work. The InfiniCaps always added an unnatural sheen and spotlighting of the treble along with making the midbass too thin and anemic. With today's better film caps like the copper V-Caps this would be even more true.
I do take exception to Atmasphere's comments about using a bypass cap across power supply filter caps. My experience with bypass caps in a PS is exactly the same as with coupling caps. I can always hear the effect of the bypass cap and it causes a loss of coherency and usually an artificial brightness and thinning of the midbass. In my opinion it is always the better course to use a single very high quality cap rather than a combination of caps.
I do take exception to Atmasphere's comments about using a bypass cap across power supply filter caps. My experience with bypass caps in a PS is exactly the same as with coupling caps. I can always hear the effect of the bypass cap and it causes a loss of coherency and usually an artificial brightness and thinning of the midbass. In my opinion it is always the better course to use a single very high quality cap rather than a combination of caps.