how can I get DSD signal from my Sony SCD5400 ES

Are you sure there's an HDMI DSD DAC on the way? :)

The 5400ES has a pretty highly rated DAC built in. You can just run the analog outputs to your headphone amp. Any SACD you play will have the DSD material decoded internally to the analog outputs.

Ah OK.

I can think of a few options:
a. get it modded by Modwright. It modifies the analog output stage and the power supply and is quite highly rated

b. sell the 5400ES for a better SACD player. I am quite pleased with my DSD DACs from Playback Designs and EMM Labs. Well worth an audition.

c. wait for an HDMI DSD DAC. The only HDMI DAC that is 2 channel and "audiophile" grade (ie not AV/home theatre) is the NAD M51. But that does not accept DSD over HDMI, only PCM.

Note the Hydra X you mentioned is a USB -> I2S converter. What it claims is that it is compatible with DOP (DSD over PCM) encapsulation over USB. This is a way that computers can take existing DSD encoded files and send it over USB to a DAC that can decode the DSD directly. It is not a DAC and it does not work with the Sony 5400ES.

Frankly I feel HDMI is not a great transport for audio signals. The M51 for instance often sounds better using the SPDIF than HDMI.