I found a fascinating article about Beyer Headphones, of which I own the DT880 600 Ohm version.  The article soon dives into impedance and damping and appears to be a good explanation of the output impedance of your gear vs. the impedance of your headphones.  

http://www.innerfidelity.com/content/comparison-beyerdynamic-dt-880-32-ohm-dt-88...

The article suggests there be a damping factor of 10 of your headphones over the output impedance.  However, it only references solid state amps.

What do you guys think of this article?  Does this apply to tube based amps as well.  (Looks like the CSP2+ is rated at <50 Ohms.)  Perhaps this is why you normally see recommendations for higher impedance phones for tube gear?