I decided to order the Western Electric tubes. I placed the order last Tuesday and the tubes showed up today (despite the message on their website to expect 30 days before shipping).

I moved the amps into my main system yesterday (Sunday), and after listening to a couple songs using my Pass XA60.8s, an audiophile friend and I swapped in the 300B SETs. The SET amps sound a bit strained at higher volumes compared to the Pass amps, but still pleasant to listen to. Vocals and guitar, in particular, sound quite nice and perhaps a touch more natural than the Pass amps. But they play loud enough that I think they will be an excellent match to the Line Forces when I get them done (Jay is still waiting for the composite material to fabricate the enclosures).

After plugging in the WE300B tubes this morning, it's obvious that these are a step up from the relatively inexpensive Linai tubes. Right out of the box, the level of clarity and sweetness in the midrange was impressive. WE says these tubes take up to 500 hours to break in.
For the last several hours, I've been listening to Norah Jones, Sophie Zelmani, and Chantal Chamberland - relatively uncomplicated music with beautiful female vocals. On this kind of music, the 300B amps sound gorgeous, even with almost no break-in. I'm looking forward to hearing how they evolve as they get more hours on them.