I have mucho experience with the THD Hotplate (and some others, but not the Dr Z). It is imperative that the box used is properly matched to the amp's output impedance. You also need to understand that as you crank up the amp, the output tubes will be taxed and their lifespan will be shortened accordingly.
I like the THD, it seems to sound more like the fully-cranked amp pushing the speakers as if it really isn't there...does that make sense? I say that because when you push the amp without a power attenuator, you are also pushing the speaker(s) as well. This adds a color to the sound that isn't duplicated when the box attenuates the power to them. The THD seems to make this tonal difference minimal. Plus their gear is built like a tank...:)

Moonbaby,

Thanks for your response and feedback. The research continues

I concur. There is a 'compression' that happens with an attenuator. But it will deliver the goods at the speaker.

We had a THD in the band in the last three years and it did a good job, but was too compressed for live performance clarity. Solution....smaller amps....hand wired...turn em up...