If you've managed to wade through this series of posts on tubes vs. transistors you'd know that while there are indeed fundamental differences between tubes and transistors that by themselves have a major impact on the sound of equipment using them, there is no single device that really does it all with perfect musicality and reliability.
For example, one might consider a tube preamplifier as the ultimate amplification device in terms of musicality but the fact is that preamplifier has many problems including microphonics (the propensity of a device to act like a microphone and reproduce what the speakers are playing), tube sonics going downhill with every hour you use it, noise, etc. Or, perhaps you've found the perfect solid state preamplifier but long for a bit more of that tube warmth or low level detail and attention to the wonderful openness tubes possess.
I hope the one thing I was able to get across in this series is that as designers we have choices. We can first learn all there is to know about the what and why each of these device types reacts the way they do, then use the advantages of both in our design.
As another example I mentioned that a voltage device always makes for a better input than a current amplifying device - but that's not always true. Nelson Pass, for example, has shown that if you take a simply bipolar transistor and run it at high voltage in a single ended mode, just like a tube, it sounds extremely musical and you get the best of both worlds. There are always penalties, of course, heat and restrictions that may not be acceptable can be part and parcel to such an approach.
On the other hand, a careful examination of each device's strengths and weakness coupled with intelligent design implementation can result in absolutely wonderful and musically satisfying high-end equipment. Unfortunately most designers seem afraid to mix and match - maybe because they don't really know all this, perhaps they just want a cleanly executed design specific to their genre or perhaps it's just easier to throw a chip amplifier into the design and call it good (lazy).
I can remember when Audio Research first introduced a sold state voltage regulator in their tube preamplifier - the howls from tube lovers the world over smacked of heretical thinking and they wondered if Audio Research had finally gone over to the dark side of solid state. Actually, they had taken a bold move to use the right device in the right application.
When designers display the courage to step outside the tried and true norms people have come to expect, magic can happen in high-end audio gear.
Tomorrow the I answer a few questions about biasing and the following day, the last in the series.