Yes, most amps are of the same basic designs that have been around for decades, and yes there have been lots of improvements to materials, passive (and even some active) components, insulation and core materials for transformers,etc, but not all these innovations are necessarily better in all circumstances.
That said however, there is still room for innovation such as the kinds of things you see coming from Roger Modjeski and David Berning -- two guys who threw out the classic tube manuals, did their own fundamental research and came up with things that push the envelope just a bit further. Nelson Pass with his re-introduction of power JFET circuits, is also a case where an active component technology has improved and a long dismissed technology brought back in a new application.
Not all these amps are going to be everybody's cup of tea, but they are all fairly out of the mainstream in terms of what's going on inside.
There is also a fairly new technology that is starting to be used in the microwave communications industry, which I predict will eventually show up in audio and will give rise to yet another whole new paradigm, even if it is still a variation on basic linear amplifier design. The technology I'm speaking of is integrated circuits with on-chip vacuum tube technology -- extremely small vacuum tubes of course (small enough to fit on a silicon chip), but still vacuum tubes nonetheless. Imagine a killer hybrid preamp that would fit in a matchbook and run for weeks on a small battery. The technology is here, even if it's mostly in the research lab phase now, but it seems inevitable that it will eventually make it's way to audio.
-- Jim
