Nelson Pass has, over the years, been trying to achieve the "sound" of a good single ended design with little or no negative feedback. He has admitted that tubes can sound very good and a SET is just about the perfect configuration for those requirements, with the exception of power. Triodes are the most linear device yet conceived while transistors usually require copious amounts of NFB to achieve linearity and their vanishingly small amounts of Total Harmonic Distortion (THD).
If the THD figures equated to good sound, then all would be right with the world, but it seems that this isn't the case as tubes have many times the THD as most SS amp and yet...
Nelson Pass has been advocating reduction in NFB, especially Global Feedback, as a key to better (more tube-like?) sound. Coupled with reduction in the number of gain stages (he's down to one stage on the recent designs) he seems to have achieved significant sonic advances with his First-Watt designs.
Tubes have been commonly faulted as having "flabby, out of control bass" and critics have been quick to point out that tubes, especially SETs have very low Damping Factors, often under 20, while SS has numbers into the many 100's.
As the Damping Factor of a SS amp is almost always derived from high levels of negative feedback, we start to see this "number" is a "byproduct" of NFB and isn't actually or necessarily a bonus. Tubes with their output transformers can almost always achieve a half octave of bass extention over most transistor designs... if the impedance of the speaker is fairly flat. This is easy to test by installing a 2 or 3 ohm power resistor in series between your SS amp and the speaker's positive input terminal. The resistor will lower the effective damping factor of the amp as seen by the speaker and you will probably gain some bass extension. Many of the best speaker designers (Greg Monfort for example), when doing a custom design will ask if you're planning on using tubes, as they take that into consideration in the overall design plan. Where tubes run into problems is with wild, fluctuating impedance curves. Please note that it's the curve that's important, not just a static number summarizing the whole complex impedance variations. As a lot of speakers are voiced using SS amps, the impedance curve tends to be forgotten as it really isn't as big an issue as it is with a tube amp. It's no wonder that tubes sound bad in many systems, as they have to contend with circumstances that are not congenial and should have been taken into consideration during the speaker design process.
Now that I've gotten that off my chest, I believe that bi-amping is the way to go with tubes for the mids and treble (they sound great) and a powerful SS amp for the bass. Yes, you should look at the impedance curve of your woofers and you can certainly apply the series resistor trick to see if you can add some bass extension with the SS amp.
Best Regards,
TerryO
