Very nice tool, thank you for this awesome freebie!
Here's a couple of thoughts:
The dropdown box for bypass doesn't allow blind A/B, as you always know which one is which. Would it be possible to have a bypass button in the graphic section, so you could close your eyes and click it until you don't know if you're listening to the processed signal or not, except by using your ears.
The matching works very well in general applications, but when the processing increases low frequency content, or boosts frequency areas where the ear is the most perceptive, the balance might not sound perfect. I think the EBU loudness algos are meant to overcome this problem, but I'm not sure how much work it would require to implement them in JS?