Round 2: SR Purple fuse vs solid titanium slug.
I wasn't sure what to expect. Titanium is way down the conductivity table compared to copper, and worse than "low grade" metals like tin or nickel. So what could it possibly bring to the table? But it's still a conductor, and absolute resistance is still quite low with a large diameter solid slug (e.g. zero ohms registered on my multi-meter, which has a resolution of 0.1 ohms).
So, what about SQ? Well, I was somewhat taken aback that I could immediately tell that the titanium slug sounded better than the Purple fuse! In particular, having greater clarity, focus and dynamics. Swapping back to the Purple fuse after a few weeks showed up the Purple to be very slightly muffled and coloured, characteristics that were not at all apparent when the Purple was beating the copper slug (or stock fuse). In my balanced hifi system (with the relatively forgiving Meze Elite headphones), this extra clarity was very welcome, but it's possible this could become too much in a brighter system, so YMMV as always.
In the A/V system, SQ improvements with the titanium slug were again rather marginal, but the OLED picture quality took another incremental step forward.
My conclusion at end of Round 2 was that the titanium slug sounded better than SR Purple fuse, which sounded better than copper slug, which sounded better than stock fuse. Which shows that the situation is more nuanced than the often black-and-white stances taken by some commentators. It also shows that, whilst conductivity may be important, it is not the only consideration for best SQ. Whilst I'd class the differences as incremental, they nevertheless affected my musical enjoyment sufficiently for me to become dissatisfied with the lesser components that I had previously been happy with.
The most satisfying part of this result for me is the cost of the titanium slug. I got 3 of them cut to exact fuse size for a total of £6. I re-purposed my now-spare SR Purple to my OLED TV, and my third titanium slug to my soundbar in my A/V system. Both PQ and SQ took a further incremental step forward, with finally a gain in dialogue clarity from the soundbar, so nothing was wasted. Roll on to final Round 3.