Instead of the Essence STX & Phoebus, get the Xonar DX or D1, same audio processor (CMI8788) as the STX
Phoebus comes with a revision of the CMI8788, the CMI8888.
You can get a used Xonar DX or D1 for around $50-$60.
Then get a Schiit Magni headphone amplifier, $99, plug it into the DX/D1.
The Magni give you a volume knob at your finger tips and can be used with more then just the computer.
Also as the Magni has a very low headphone output impedance (less then 1-Ohm), it does a little better job at driving lower Ohm headphones.
The SB-ZxR (Sound Blaster ZxR) uses the same headphone amplifier (TI 6120) as the STX & Phoebus.