If you look at the graph George posted, be sure to observe the vertical scale as it is in fractions of a dB. -0.8dB down is not bad for a transformer coupled unit. However it should be noted that output transformers need to be loaded properly for optimal response. Given that PS Audio would not know what the unit was driving, its likely the loading is internal in the unit itself. This would mean the external load of an amplifier is likely irrelevant.
From the same Stereophile review:
From the same Stereophile review:
In each case, the audioband response has a very slight rolloff in the low bass, reaching –0.8dB at 10Hz, but is otherwise flat up to 20kHz.