The pop is normal if you do not put it into bypass and wait a few seconds prior to shutting the power off. This is not damaging anything.
My DT25 also has a hum. There are a lot of reasons that could cause the hum. Does it change with the voicing(digital preamp model)? I note you attempted different power amp settings. On mine the models had varying hiss depending on the model used. Line6 simulated the hiss and spit of the preams the mimiced. Voicing 3 seems to be the quietest for me. Setting mine into triode mode produces a low hum vs a higher hiss with the pentode. Is the hum master volume sensitive? If so the issue resides in the preamp. If not look to the power amp side. The DT25 is very picky about constant noise free power so borrow a power conditioner if you can and see if that helps. My mods to reduce hiss were replace the stock tubes with MESA tubes and rebias. I also bought a Furman M-8 line conditioner(eliminated random crackles and pops). Some hiss/hum is normal in tube amps. I tested the DT25 side by side with a MESA TA-15 using the same speaker cab in a isolation room at Guitar Center. I found that the hiss was comparable for similar topologies. I did note that the TA-15 sounded subtly brighter and more punchy due to pure analog path but, cost and lack of channel switching killed it for me.