Eric,
Both devices are very popular and great for audio. I can't comment necessarily on the sound comparison. The OPA1656 is a newer device with better typical room temperature specifications in regards to the following:
| Test | OPA1656 | OPA134 |
| GBW | 53MHz | 8MHz |
| Slew Rate | 24V/us | 20V/us |
| PSRR | 0.3uV/V = 130dB | 106dB |
| CMRR | 120dB | 90dB |
| Ib | 10pA typ 20pA max | 5pA typ 100pA max |
| Aol | 150dB 600 ohm load | 120dB 600 ohm load |
Stability should be considered especially with the wider bandwidth of the OPA1656.
The output swing for Aol should be considered as it is different between the two devices. For example, Aol for the OPA1656 with a 600 ohm load will have a linear output swing that is 1.3V inside of each supply. Aol for the OPA2134 with a 600 ohm load has an output swing that is 2.2V inside the negative rail and 2.5V inside of the positive rail . If the customer is not swinging the signal this high on the output then this consideration doesn't matter but I should bring it up as a consideration.
The OPA1656 does have more output drive capability as seen in the claw curves below.
OPA1656
OPA134 family
Both devices have great noise and THD specifications.
