While you're correct about driving the transformer primary from a low source impedance (less than 10% of the transformer's primary DC resistance is the point of diminishing returns), there's a caveat. An output transformer may have considerable distributed capacitance (effectively in parallel with primary) and it can destabilize the driving output stage. Using the series resistor on the primary prevents the driver from experiencing the effect at near-Mhz frequencies. A better technique is to use a ferrite bead or parallel L and R on the primary side (3 uH and 39 ohms is common). A ferrite bead with an impedance around 50 ohms at 1 MHz usually works well ... and the impedance of either method at audio frequencies is near-zero! Jensen may still sell the JT- OLI output isolator, which winds the inductor on the body of the resistor.
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