I'm not sure I agree, especially when you factor in substantial equity. We have interviewed numerous people from banks and large tech companies and our salaries certainly don't seem out of whack.
To you, that "substantial equity" might be a real cost, but to the applicant it's essentially a lottery ticket. The expected value of your equity is zero. I've worked in many silicon valley firms that offered stock options but the only time one ended up having any value to me was as a bargaining chip - I was able to tell the next firm that wanted to hire me "gee, I'd love to take your offer but if I leave now I'm giving up all these unvested options worth lots of imaginary money" which helped negotiate a signing bonus.
Most companies fail, making their stock options worthless. And even the few companies that succeed usually hit bumps in the road that dilute the stock options or put their value underwater.
So if you have a reason to think your idea of "substantial equity" is much more likely than average to ever be worth anything at all, you should make that fact a key part of your pitch. (Or if you're indifferent between equity and salary, offer more salary and less equity.)
"especially when you factor in substantial equity"
Unless your company ends up being the next Google or Facebook (which is a statistically unlikely outcome for any startup), your "substantial equity" may never be worth any significant amount of money. Living in NYC is expensive, and you can't pay your rent with stock options.
Maybe the AngelList data is why startups have trouble finding good engineers? There is no good way of finding out what a good developer should get, and that is because the companies (including yours) like it that way.
I only have anecdotal data, I don't think many people have more, but I am very sure a decent senior dev can get way more than what you are offering.
I think there is more to a job than just the salary, but at least be open about the fact that there are open positions doing similar work in the same city offering significantly more.
Banks will pay at least 2x more and large tech companies closer to that than what you are offering.
Note I'm not talking about truly great developers, merely good senior ones.