A great amplifier when it works.
I have since bought a replacement EH6922 tube to test, and it turns out that my imbalanced noise floor issue (hissing) appears to have been a defective tube. The new tube also "snapped" into the socket instead of being a steady pressure until the tube was socketed, so I am unsure if this makes the difference or not. I now have had a very slight return of the intermittent electronic buzzing that i experienced with the first amp, but I will keep monitoring and this may just be a compatibility issue with my HD 700. I have added 2 more stars to my review, as the amp is now usable again, but am still very disappointed with overall Drop support for their manufactured devices.
I really enjoyed the CTH, and it paired very well with my Sennheiser HD 700, imparting some great tube coloration with plenty of power. The only issue is that quality is just not there, and my CTH's have malfunctioned permanently after about 6 months. I have been through 2 CTH units, and have had showstopping issues with both. My first CTH developed an intermittent loud clicking/buzzing in the left channel approximately 6 months in, and was replaced under the default 1 year Massdrop warranty a few months later as it got worse. My replacement CTH then developed a running water style hiss on the right channel, this time before the 6 month mark, which has gotten progressively louder to the point that I no longer want to use it. Unfortunately, no warranty is provided on replacement units beyond 30 days. Judging by the majority of reviews, I may have just been unlucky enough to get 2 lemons in a row, but with the lack of warranty support I cannot recommend the CTH anymore. For the price, 6 months of ideal functionality is just not worth it. Maybe the solid-state amplifiers are more reliable, but without warranty support I am not sure the gamble is worth it. Very limited warranties are not unique to Drop, and I assume I am an outlier with my experience, but it is a factor to be aware of.
Would not recommend to a friend.