[Otto]

Hi Rod,

Since you started to have a problem with more than one of them, is it possible that it's something else in the signal path? Perhaps your amp/receiver is still sending a low level signal when there's no obvious audio signal (you could see this with an oscilloscope, if you have access to one). Or perhaps there's some other type of interference (I took a guitar amp to the shop once because I was hearing weird sounds from it -- turned out to be cell phone interference). With these types of things, I wouldn't be surprised if one speaker goes bad over time, but to have several go bad in the same time frame would send me looking somewhere else.

Is this sound coming from the powered sub? Or is it coming from the other drivers?

If it's coming from the powered sub, I would say that it's possible that it's the DT that's the problem. If it's coming from the main speakers (and this may not be obvious; have a close listen), then I'd say it's a problem with the source or amplification chain -- the drivers (other than the sub) in the DTs can't do anything on their own, so the problem would be in the amp or signal that's driving the amp.