I've never heard something that measures well sound bad - however the opposite definitely can be true. You can measure anything ... though I suppose I would say that because I work for a measurement company. Still not sure I believe in opamp sound. Noise yes - that's why I would use an expensive one, rather than sound qualities. In a buffer the contribution of the chip will be more than 100db down at audio frequencies - measurable? Just, but extremely hard. Audible? No chance. Chips are probably chosen based on cost, supply requirements, output current, noise performance in that order for audio. As ever, to each their own - but expecting miracles from swapping opamps will of course make you hear miracles. I don't expect it to make a difference, so I don't hear one.