Interestingly there is a correlation between this and guitar playing. I have several times joined the local BMW car club to first go to their performance driving school and then to participate in a couple of their track days. I did the performance driving school in the bmw I owned at the time...
I did the Watts Valve Jr replacement board and output transformer on my Valve Jr combo. Less complicated than the whole Mercury Magnetics upgrade but still a much cleaner sounding amp.