Good list, Dave. Although you already know that I disagree completely with your comment about old valves. I find good old valves so superior to JJ's and any current production valve that I doubt that I will never use a current production again. And no more expensive than modern valves once you've educated yourslef a bit. It does also depend a lot on the amp circuit though. Some amps will benefit more than others with good valves.
I'd also like to add another point which I have been very guilty of at times, which is ......
......don't forget to adjust your amp's EQ knobs, even if you think you've got your amp sussed. It can make a huge difference between very poor and outstanding tone. I have been guilty many times of moaning about my poor tone all night, and yet forgetting to try adjusting my amp's EQ. I had made an assumption that, as I knew the amp well, I had the EQ set perfectly already. I guess it's just that I'm a bit lazy really, and very stubborn !
I am now realising that each venue might well need completely different settings.
Which leads onto my last point ...... remember that guitar amps/cabs can sound different from day to day, dependant on acoustics, electrical supply, electrical interference, and sheer sorcery !