popped up in one of my trade publications for semiconductors. maybe one of you smart guys can summarize for the rest of us

http://electronicdesign.com/article/...n-Sand-Part-1-

"Conclusion

I wanted to delve into the subject and try and explain some of the sound differences that were wrongly perceived as being purely related to the vacuum tube in both the purist extreme application and that of live audio and guitar amplifiers. We proved that the active devices are a fairly small portion of the observed audible differences by a drop-in experiment, which led to investigating the rest of the signal path and the output damping factor.
You can easily approach the “tube-amp sound” by simply placing a substantial resistance in series with the output of a modern voltage-source audio amplifier. This decreases the damping factor and allows the bell modes on the transducer cone and dust cap to radiate as opposed to being clamped by an output impedance that is effectively zero."