Hi Newbee,
I agree that a failure of either a tube or solid state preamp can conceivably destroy speakers. DC won't destroy tweeters, though, since the crossover network in the speaker would route it to the lowest frequency driver(s). Unless, that is, its onset is extremely fast, and it is extremely large, in which case I suppose it is possible that a transient at the instant of that onset could be fast enough and large enough to be routed to the tweeters and cause damage in that instant. But that seems very unlikely.
Also, most tube preamps will not be able to output DC since most of them have coupling capacitors at their outputs, and most of those that don't have coupling capacitors have transformer coupled outputs or some other means of DC protection instead.
A relevant anecdote: A few years ago I was listening to my system just after installing the VAC tube power amp I still use, which came with a brand new set of Chinese tubes. After about two hours of beautiful music-making, from one second to the next a huge outburst of continuous static suddenly erupted in one channel. I had the amp shut down within about four seconds, with no secondary damage occurring to either the speakers or the amp. I subsequently determined that a 6SN7 small signal tube in the amp had developed a short. It does seem very conceivable to me that that kind of high volume static, which an output transformer would (and in this case did) pass pretty much unimpeded, could destroy the tweeters in some speakers, especially if the user was not nearby and able to shut things down promptly. And depending also on the power capability of the amp.
In that case, as I say, the problem was a small signal tube in the amp, but the same thing could happen just as easily in a preamp.
In general I would agree with Schubert that the chances of a solid state preamp causing damage can be expected to be significantly lower vs. a tube preamp, but I have no particular feel for how much lower.
Best,
-- Al
