Interested in hearing about tube rolling experiences with the Orchid and other MHDT dacs, namely, the specific comparative sonic characteristics of the various 396a, 5670 and other compatible tubes used in the output buffer. My only experience is with the WE396a that Linear Tube Audio installed in my new Orchid.
@pmboyd   do you have a link to a Siemens 6922? Thanks. 
gelle... Siemens made valves are from German and the European 6922 (American designation) equivalent is the e88cc. Many tube dealers, such as Brent Jessee, carry them. Top of the Siemens line is the CCa, followed by the e188cc (American 7308), the e88cc and the ecc88. Does that answer your question?
PS, Generally speaking, the earlier production Siemens valves -- like many other brands -- are superior to the later production.
@pmboyd   Thanks. Yea I have bought from Brent in the past. Got my WE 396 D getter from him. Looks like he is out of stock on most of the tubes you mentioned. He has some of the E88CC / 6922 Siemens, West Germany made, 1960s wide getter support post, gold pins in stock. Might try those. Need to see if the “ the Getter support post” is a wider tube than what I’m currently using and if it will fit (with adapter) into my Orchid. 
gelle... other dealers will have the Siemens tubes and they regularly come up on Ebay. "Farawayfinds" on Ebay has an excellent selection from the 6922 family. I have the Orchid as well and all the flavors of 6922 fit comfortably with a pin adapter. The Garage17 adapter is highly recommended.