Anyone's opinion about VTL amps?

If the driver tubes affect the sound of the 450 as much as the 750, I would suggest swapping the original Sovtek tubes for something with less distortion and wider bandwidth.

I have not completed my testing, but Brimar and Sylvania Military grade in 12AT7 are excellent. Also, high frequency response was increased, soundstage brought in focus and distortion decreased by replacing the supplied power cord with Purist Dominus. I suspect Elrod, Nordost and Purist would all be excellent choices for this amp, but have not tried them all.

Let us know how it works out if you experiment.

Sorry, I have not heard the Audiomeca Mephisto so I cannot know if that is the issue.

I assume you’ve tried various feet under the Audiomeca, such as the Symposium Roller blocks? The source end of the system, particularly the CD player and preamp have a great deal of affect on what comes out of the amp.

As for power cords, the Purist is more detailed than the Elrod and less compressed. The price for a pair of input tubes and a set of Roller Blocks is less than the Elrod or Purist. I would consider the less expensive auditions, working my way up as necessary to reach your goal.