I agree the effects are probably minor. I like to think of it as being able to see the bottom line of the eye chart rather than the line above when I use the LM4562. At my age I should be happy to see the line above so the extra acuity is quite seductive! Unfortunately the price is a touch of glare and I switched back to the JRC2068 this morning to calm things down.
To my ears there is not much difference between the JRC2068 and the NE5532. I am pretty happy to use the JRC5536 with a gain of 2 as the preamp for my speaker system so I might try that too. Its low bandwidth could be an advantage.
At the other extreme I am curious to hear how the O2 sounds without the gain amplifier. Hopefully the ODAC can drive the paralleled load resistor and volume pot and there is enough volume from my 32R phones.