It WILL NEVER be as good as tubes,class a or good a/b NEVER!!!
As a tube amplifier manufacturer I have to disagree with this comment.
We are a tube amplifier manufacturer that is also developing a class D amp.
Our tube amps have gotten excellent reviews and numerous awards in the high end audio press; in particular they are known for their transparency and musical nature. So I'm not making this statement lightly.
No-one knows the future but we do know that people make innovations. We made one and its why we have a patent application filed on our class D amp; I am of the opinion that class D has a lot to offer in the transistors vs tubes debate. IOW, I wouldn't say 'NEVER'...
There is a thing called a 'technology/performance' or 'innovation' curve. Its generally the shape of an S. At the beginning, when the technology first appears, a lot of time and money goes into its development, with small increases in performance. As the tech gets sorted out, with small amounts of time and money much larger performance increases occur; then as the tech is mature (think transistors and tubes here) then larger amounts of time and money yield incremental improvements.
Class D is in the middle of the curve, so its still possible that smaller companies like mine are able to make big improvements. There are a lot of players in class D so 'never' seems dubious to me.