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Not that its badly implemented because its not. What I mean is adding an amp won't magickly give it a better snr . So the DAC is always the limiting factor. If he needs more power for headphones then yes it will improve his headphones sound. I'm speaking strictly of the snr.
Going from the z to zxr netted pretty big sq gains for me. Remember the z's Headphone port is bottlenecked already by the maxxim amp after the DAC. Roughly 109 db snr. Not the 116 for line outs. The Headamp in the z is definitely mediocre/ok. But not bad at all. Was able to power DT 770 250 ohms with no issue.
I am not sure you know what 116 dB SNR (assuming it is not just a marketing figure not actually achieved in reality) really means. For that to produce audible hiss, you would need more than about 125 dB peak SPL (which is very loud, and would be too much for most people) even in a fairly quiet listening environment; even then, it would only be audible while no music is playing. Additionally, 16-bit CD quality music (which is something like 99% of the music currently sold) has only about 96 dB A-weighted dynamic range with typical simple dithering; adding a -116 dB noise floor to that would only make the overall noise level worse by a few hundredths of a dB, which is insignificant.
In other words, the SNR of any decent DAC is usually not much of an issue in practice; that is, assuming that the DAC is running at or not much lower than 100% digital volume, and the volume is mostly controlled downstream of the DAC (this is not the case with the amplified headphone outputs of sound cards like the Xonar Essence STX). At the headphone output, listening at a realistic level, the SNR of the amplifier can easily be much worse than 116 dB.
To get an idea what various levels of noise sound like, try
this link, where you can compare a 24-bit sample to various quantized versions from 16-bit to 8-bit. The 16-bit sample has a dynamic range of about 97.3 dB (A-weighted), and losing one bit of resolution makes it worse by 6.02 dB. There is another test to try
here, where short samples of various music have been recorded from a number of devices, including the Xonar D1 (~110 dB SNR at 44100 Hz sample rate), and even Realtek onboard codecs.