I understand your concern regarding the use of tube amps, especially OTL tube amps with the Denons, but I beg to differ. I own a Fostex TH900, but it is nearly identical to the D7000 in terms of specs (impedance, sensitivity, design, sound signature, etc.). So let's take the D7000 for example.

I believe your rationale is that the D7000 is a low impedance headphone, so one would need an amp with low output impedance (which most solid-state amps are but tube amps aren't) to ensure enough damping factor to control the drivers and reduce distortion. I completely get that. However, there's something special about the D7000 (and the TH900, for that matter). We say the D7000 has an impedance of 25 ohms, but to be precise, it is 25 ohms at 1kHz. The reason we use the term "impedance" and not "resistance" is because headphones have varying resistant value depending on the sound frequency delivered. This is what impedance means, and why we need to consider looking at the impedance curve. In the link provided below, the impedance curve is the second graph from the left.

http://www.innerfidelity.com/images/DenonAHD7000.pdf

As you can see, the impedance curve is mostly flat, and the highest at 40Hz, at around 31 ohms. So the impedance variation would be 25 to 31 ohms. Compared to the HD800 (we are told it has an impedance of 300 ohms), which fluctuates between around 300 to 600 ohms, the D7000 can be said to have a very flat impedance curve.

http://www.innerfidelity.com/images/SennheiserHD800.pdf

And it is this variation in impedance that causes impedance-related distortions. The general acceptable amount of distortion, in other words variation, is 1 dB or below, which the human ears cannot discern. The formula for calculating this variation in dB is as follows: 20 * log[(Zmin+Zamp)*Zmax/(Zmax+Zamp)*Zmin], where Zmin is minimum impedance of headphone, Zmax is maximum impedance of headphone, and Zamp is the output impedance of the amp.

This result of this formula should be 1 or less to ensure there is no audible distortion caused by improper matching of impedance between headphone and amp. Due to the nature of this formula, the result becomes close to zero when 1. there is a large gap between Zmin (or Zmax) and Zamp, 2. there is a little gap between Zmin and Zmax, 3. the Zamp (output impedance of amp) is close to zero. Theoretically, if the impedance curve were ideally flay, meaning Zmin=Zmax, there would be completely zero distortion no matter how high the output impedance of the amp is. In other words, if the D7000 had a constant resistance of 25 ohms throughout all frequency range, you could connect it to a 100 ohm-output impedance OTL amp and still have no distortion whatsoever.

Because the Zmin = 25 and Zmax = 31 in D7000's case, there is a little gap between Zmin and Zmax. In such cases, the need to have a smaller output impedance of the amp becomes less important. If we were to drive it with a tube amp whose output impedance is 5 ohms, the result of the formula would show that the maximal distortion is only about 0.3 dB. At output impedance of 10 ohms, the distortion is about 0.5-0.6 dB. At output impedance of 30 ohms (which actually happens to be the "low" output impedance of WA6SE, your favorite amp with the D7000), the distortion is still around 1 dB or less.

So, there you have it. Although I would still advise against OTL tube amps whose output impedance are 50 ohms or higher, the D7000 is likely to work well with a wide range of transformer-coupled tube amps whose output impedance are 30 ohms or lower. Of course, there are other, much more important factors in proper matching of amp and headphones, but what I'm saying is that one would not need to worry about using tube amps in general only because they have higher output impedance than solid-states. If the D7000 had a very fluctuating impedance curve like the HD800, I would stay away from tube amps too. But as I've proved in above paragraphs, the relatively flat impedance curve of the D7000 allows it to be driven by many transformer-coupled tube amps without impedance-related distortion issues. If the D7000 sounds bad with a particular tube amp, it should be not because of damping factor but because of something else. Same goes for solid-state amps.