I suspect that your problem lies with your amplifier.
If I recall correctly, the Maggies prior to the ".7's" had rather ruler flat impedance curves.
The crossovers have changed and this is no longer the case.
I owned MGII B's, 3.6's and still have a pair of early MMG's.
The MGII B's (6 ohm load ) could be driven with a 40 watt PP tube amp quite nicely and sounded beautiful, at low to moderate levels.
The 3.6's I drove with a Classe CA400 SS amp which pumped out 800 watts into a 4 ohm load.
You tried a second tube amp and still had the same issue.
I would try all of the output taps on the Ref150 and see if there is a difference. ( without the auto formers first )
I would then try the auto formers with the best performing tap if you still need them.
My experience is that Maggies need to be driven at moderate levels at the very least to sound best.
More power was always better for me with Maggies.
The Ref 150s may not be up to the task, even with the auto formers.
If you have not visited the Planar Forum at the Audio Asylum, you should and ask for assistance there.
If I recall correctly, the Maggies prior to the ".7's" had rather ruler flat impedance curves.
The crossovers have changed and this is no longer the case.
I owned MGII B's, 3.6's and still have a pair of early MMG's.
The MGII B's (6 ohm load ) could be driven with a 40 watt PP tube amp quite nicely and sounded beautiful, at low to moderate levels.
The 3.6's I drove with a Classe CA400 SS amp which pumped out 800 watts into a 4 ohm load.
You tried a second tube amp and still had the same issue.
I would try all of the output taps on the Ref150 and see if there is a difference. ( without the auto formers first )
I would then try the auto formers with the best performing tap if you still need them.
My experience is that Maggies need to be driven at moderate levels at the very least to sound best.
More power was always better for me with Maggies.
The Ref 150s may not be up to the task, even with the auto formers.
If you have not visited the Planar Forum at the Audio Asylum, you should and ask for assistance there.