You'll have to address that question to the Elecraft design staff.
My guess is that it was to protect the DAC's output amplifier from short
circuits a user might inadvertently apply across J22, the LIN OUT port,
but that can be done with much less resistance.
I can't speak on behalf of the designer either, but more often that not, a
series build-out resistor equal to the specified primary Z is oftentimes
seen in professional audio applications when driving the transformer from a
near zero Z source -- like an op-amp. *Some* designers feel an absolute
need to match the source Z with the transformer primary.
In reality, the best value for Rs is what produces the best compromise
between distortion, transient response, protection to the driving source,
noise, and secondary output level.
Speaking of transient response, I have ordered the TTC-108 and Triad TY-145P
transformers and will be testing them against a high quality Jensen type.
High quality transformers like those made by Jensen are generally uniform in
phase and amplitude as a function of audio frequency. A Tektronix TM series
audio test set will be used compare square wave response under varying
degrees of source and terminating Z across all three transformers.
A square-wave test through a transformer can be important in this
application since an improper termination of the secondary can lead to
severe overshoot and ringing into relatively Hi-Z bridging devices like a PC
sound card. While sine-wave level testing may appear to be acceptable,
bizarre results can occur when trying to feed a demodulated CW signal (a
square wave-like source) into a PC sound card for applications like CW
Skimmer.
Paul, W9AC
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