Solved this way, though idk if its the most elegant solution. Exported a nice full-sized 600×600 png from illustrator. Dumped it into the Header Logo position. Added the following to Custom CSS:
#site-logo {
max-width: 75px;
}
And now it looks all crisp and lovely. I tried putting out 75×75 png from illustrator but it still looked blurry, like it had been scaled up and then back down, or something. Tried using Media Library to scale the 600×600 image down to 75×75, and still blurry. for whatever reason scaling it this way with css looks about 1000x better. if there’s a simpler, more logical way to fix in WP, or export from AI, or whatevs without editing the CSS, please share!