Like a number of other PrimaLuna amps your Prologue Sevens have a particularly high output impedance. Stereophile measured 2.6 ohms for the 2 ohm tap, 4.5 ohms for the 4 ohm tap, and 8.5 ohms for the 8 ohm tap. The impedance of your speakers, which is similar to the impedance of many Martin Logan speakers, is spec’d at 4 ohms nominal, 0.8 ohms at 20 kHz.
The result of the interaction between those impedances is undoubtedly a considerable amount of rolloff in the upper treble region, regardless of which tap you are using. (The amount of rolloff would be least on the 2 ohm tap, although using that tap with nominally 4 ohm speakers might compromise the maximum power capability and the distortion performance of the amp).
Going to the 11a, btw, would probably be even worse in that respect, as it is spec’d as having an impedance of only 0.6 ohms at 20 kHz.
So yes, changing amps is likely to make a big difference, and probably for the better from a subjective standpoint as well as in terms of accuracy.
Good luck. Regards,
-- Al
