Just picked up a F2a amp in late November. It was built by Sean Casper who is a DIY builder in Wisconsin. I've been interested in the Siemens F2a tubes for a while as they are famous for their use in old Klangfilm amps of the 1950's that were used in German Cinema.
The roughly 8W F2a has tremendous dynamic punch and very good bass for a SET. It also has wonderful presence which reminds me a little of my much more powerful GM70. What is it missing? Well, in comparison to my 300B amp it is missing some midrange sweetness and a little bit of nuance. However I am glad to add this amp to my stable. It is a wonderfully musical tube that deserves a wider audience.