I do have ant-imatter here, the blighters have a fetish for taking the entire bivouac into power boxes & power points & the old Bakelite switches (must love the smell of Formaldehyde). This normally causing problems. Surface spray is your friend.

It is rare to see a valve coupled directly to a " sound reproducer" in latter years, but headphones & early speakers (glorified headphones) were in the DC path. In most cases in latter years it was a low impedance speaker transformer coupled, it providing DC isolation and impedance matching of the load.

Transistors of course go back to the old way and there is often no transformer: Then polarity is then critical

Of interest to this post is 6BM8. The link is to a circuit where a capacitor couples to the speaker.

http://frank.pocnet.net/sheets/127/6/6BM8.pdf

Yes, but there's a choke in the plate circuit (similar to a single ended transformer primary, with air gap) and the load impedance still needs to be about 7k ohms.

That would be useful if you wanted to run the speaker wires a long way with a transformer on the speaker and still have it ground referenced.

Very much like a class A transistor output stage as was commonly used in early hybrid car radios. But in this case impedances are lower so you can couple direct to a speaker.