Hi, I have had experience with Siemens tubes. I actually prefer the 7308 version to the 6922. I have found Siemens tubes to be very open, detailed, and transparent. I previously had a CAT SLl MK lll preamp, and the Siemens 7308 was the best sounding and most durable for the phono section.
I have found through extensive tube rolling, that each time I change a component, I find myself trying every version to get the best sound for that particular piece of audio gear.
There are several tube sellers on Agon, and I have found them to be very honest and knowledgable about the NOS tubes that they sell.
I can highly recommend an internet seller that stands behind everything he sells. He even goes as far as accepting returns if the particular tube you purchased doesn't sound right for you piece of gear. You just can't beat that!
Google Brent Jesse Recording. He has a great website that is very easy to negotiate. Prices are fair, and his communication skills are incomparable.