"""""""By the way, I was trying to fillet the join at the base of the headphones where it goes from an oval shape to a round shape (which I made with a blend), but I didn't seem able to do that. Not sure what I am doing wrong there, so any tips on how to make it work are appreciated.""""""

a "Blend" would not have any joins that would be/need a fillet. The connections would be tangent. That looks like it was made with a "loft" which just made a straight shot from the oval to the circle, but leaves a hard edge at the oval connection. The fillet there will be tricky and probably have one side look creased, but the other side be more tangent (no fillet).

So you CAN run blend there, to get a smooth transition. First, capture the ending circle by doing a drill down selection (click the object, then do a second click on edge (circle) and copy and paste it. Then do a drill down selection on the surface that goes from the oval to the circle and delete it (You now have an open edge on the oval side) Do and extrude of the circle with no caps. Now you can blend the edge of the oval and the edge of the circle. Delete the extruded circle surface and cap the end of the blend and join the surfaces.

It helps with stuff like this if you provide a 3dm file of the model to look at exactly whats going on....