Sorry Atmasphere, I didn't mean to imply that CMRR was the raison d'etre for your inclusion of the termination resistor. It usually only shows a handful of dB improvement for the CMRR in practice, sometimes a little more at higher frequencies. A certain REL subwoofer input circuit comes to mind, and this is a single-opamp differntial input stage.
I do think that it's highly inaccurate to characterize line output transformers, as a group, as requiring a substantial or specific load on the secondary to acheive proper transient response. While there have certainly been vast quantities of poor-quality transformers produced over the decades, tolerance of a wide range of secondary load impedances is one of the fundamental differences between output and input transformers. On a modern line output transformer, it is in fact the tight coupling between the windings and their higher inter-winding capacitances that allow the primary to provide damping for the secondary. This of course requires that the primary be driven from a very low impedance source, as indicated on transformer datasheets.
Sonically, I feel that the best input transformers are virtually transparent, with the main audible artifacts a result of huge, measureable improvements in bandwidth control and noise rejection. For output transformers, I think I usually hear do have a very slight discernable signature, but it tends to be mainly at low frequencies, and frequently a rather welcome coloration. But one example of the astounding potential of output transformers can be found in any of the Audio Precision analog-based generators . . . maybe as the patents for this famous circuit near expiration we might see something similar in an audio product that we actually listen to.
