Over the last three years Art Dudley seems to have been inconsistent about one issue concerning amplifiers. He seems to have unaccountably changed his views on what the best material is for their cases.
In the April 2004 and May 2004 and March 2005 issues he sang the praise of amplifiers from DNM. One their most notable features is that their cases are made of plastic. DNM's designer believes that even aluminum cases have deleterious effects on the signal. This is because he believes even the aluminum interacts magnetically with the signal. Presumably he would believe steel, a more magnetic material, would have even more adverse affect on the signal.
Dudley was quite enamoured of the DNM 3C preamplifier. In March 2005 he wrote that it is "simply one of the finest preamplifiers I've ever used." This was high praise for a $3495 preamp. He also wrote, "I don't doubt for a second that the DNM preamp's low mass has something do with how good it sounded."
However, in the June 2007 issue he was quite fond of Shindo's line of preamps and amps even though both their enclosures and their interior walls are made of steel. "All Shindo preamps and amps are built into steel enclosures, because Ken Shindo dislikes the sound of aluminum."
In the August 2007 issue Dudley sings a similar tune. Besides a clear acrylic top, the "rest of the chasis" of the Audio Valve Eclipse, "is crafted from laser-milled stainless steel-generously lacquered-which Becker uses simply because it sounds better."
It is hard to see how steel could be a better material than aluminum. Steel is quite magnetic and unless the signal is kept away from it inside the amp it would seem it would adversely affect the signal. The only benefit I could imagine is that perhaps it does a better job than aluminum of shielding the signal from Radio Frequency Interference (RFI). However, since the Audio Valve has a plastic top this would not help that model.
Anyway, I think Dudley ought to decide - does he believe in the benefit of plastic enclosures or does he believe in steel? Then he should be consistent.