1N5291 is rated at 100V, with ±10% tolerance. J503 is rated 50V, with ±20% tolerance. That's the reason why these dedicated CRDs are more expensive.
50V is the minimum voltage rating that should be used here, so 2SK170 cannot be used to build a CCS. 2SK30A is rated 50V, but you need to choose from a large assortment of R grade parts because discrete JFETs have a wide spread of Idss from unit to unit (can be anything from 0.3mA to 0.75mA, which corresponds to +34%, -46% from the target 0.56mA). Also, you'll need to mount the 2SK30A in reverse than marked on the board.
The Zobel network should not cause any harmful effect within the audible frequency range.
liuchengdar, your power transformers are too close to the amplifier boards. You may not hear hum/noise with your particular headphones but I am sure the noise floor spectrum will show AC fundamental and harmonic spikes.
Here is an amp's noise floor with the power transformer far away:
Here is one with the transformer nearby:
If you use low-impedance, efficient headphones you will hear the noise.