I like the sound of the Siemens, but at least in my LF, the tubes I bought emit a lot of microphonic noise. The noise ranges from a low level haze that clouds up the sound, to a high-pitched ringing. I bought 2 sets of tubes from 2 different sellers, and both have the same problem.
The noise goes away temporarily if I tap either the 1st tube (if the noise is in the left channel) or the 2nd tube (if it's in the right channel). It's really annoying. I ordered the EAT Cool Dampers that dB mentioned a few posts back, so I'm praying that those will fix the problem. The cheap tube rings from tubedepot.com don't seem to do much, but the ones I got are also so loose when fitted on the tubes that they fall off easily.
It seems that if you can request tubes tested for low noise, you may be able to minimize the risk that you'll have the same problem. Or else, the problem is in my particular LF unit. I haven't heard the noise with the stock tubes, which can actually sound clearer because of this issue.