Re: Question about Blackstar HT Club 40 (and amps in general)
No, it's not all-valve. No, it doesn't matter.
If I understand it correctly, the valves in the HT Venue series are both used as gain stages. I know the HT-5 has some diode clipping in the overdrive circuit, but I don't know if that's the case in the Venue series.
Your question about the third tube in other amps is a bit more complicated than I think you realize, but we'll take the example of a JCM800 preamp as an example. The first tube is for either one or two gain stages, depending upon whether you use the low input or the high input, and the second tube gives you another gain stage. The second stage of the second tube is used to give current drive to the power section, if I remember correctly.
The third tube, which is replaced in the Blackstar amps with solid-state, is the phase inverter. It takes half the signal and reverses the phase by 180 degrees and passes it on to the power section.
Now that I think about it, no good can come out of this answer, because I'm not even close to the authority on this, and the OP will never read it. I am in the running for most useless post of the year, though.