Found the thread discussing tube amps. This is a good post from member Pauln on the thread entitled, "Heresy light on Bass" The entire thread is here.
http://www.audiokarma.org/forums/index.php?threads/heresy-light-on-bass.671032/#post-8952712
I copied the specific part in which Pauln talks about tube amps and Heresy.
"Tube amps use output transformers; the impedance curve for the Heresys has a low of about 11ohms in the bass and has a rising elevation in the highs topping out around 70ohms.
That rising impedance elevation has a calming effect on how the horns respond to a tube amp - it is a general characteristic of the tube-horn combination and does tend to present what sounds like an overall shift from the relative brightness one may hear from using some solid state amps.
The 11ohm low means that the amp will not strain to apply bass power to the speaker... an easy load.
As far as more bass from tube amps; they are different from solid state. The SS amp output has close to zero output impedance whereas a tube amp's output impedance will be a few ohms... this is where the damping factor comes from. Tube amps have low damping factors so the woofers are not as tightly controlled. In some speakers that already have a "tight" woofer like the Hersey (low excursion, higher bass roll off, sealed cab without port) this sounds like increased bass response.
Also, in a tube amp, the output impedance seen by the tubes is the load impedance of the output transformer (OT), and the output impedance seen by the OT is the load impedance of the speakers... the OT is doing a lot of things between the tubes and the speakers that results in the "tube sound".
In a lot of electronics, transformers are just a necessary utility component providing the needs for the more exciting components to do their thing. In tube amps, the output transformers are typically the most expensive and highest quality (and heaviest) components of the amp - very much the heart and voice of the amp."