@williambf , your result would imply a difference in output resistance between the two models. Have you tried using different taps to see how you prefer the sound? I gleamed this from Stereophile, not sure they are your exact amplifiers.
This is 2012, may be similar.
As expected, the Ref150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm
This appears to be a 1994 review:
The VT-150's output impedance was nearly 1 ohm (0.99 ohms) at any audio frequency measured at the 16 ohm tap. This value decreased to 0.56 ohms at the 8 ohm tap, and 0.3 ohms at the 4 ohm tap.