There really isn't any magic here. Unless there is some significant engineering/electronics breakthrough, a very good design is a very good design. Typically I have found that manufactures do one of two things. They base their products on reviews from magazines and since magazines rarely if ever review older products (even if they are still in production), to get their names in the magazines, manufactures simply come out with a "new" product. It really isn't new or better, but a cosmetic change or a change in the type of capacitors, etc. The other thing that they do is come up with an outstanding design in the first place and stick with it. Yes, you can tweak a product over the years and make it better. Use different tubes, or different bias circuitry or different transistors. But a current mirror is a current mirror, a differential input is a differential input. I haven't seen really any major design improvements in years. Boulder simply makes great equipment. So does Audio Research, Pass, etc. and most importantly, these type of manufactures will maintain their older equipment and not leave you hanging. Audio Research, Boulder, Bryston, Pass and others will maintain their older equipment. That is very important to me.
To see how important that is, look at the resale value of used equipment. If the company continues to maintain their older, well designed equipment, the price is constant and stays up.
enjoy
