I've had some feedback from someone who bought a GPSDO that the sine output isn't as clean as I thought, and it turns out that the reason why it seemed cleaner to me was that I neglected to take into account that I have only a 50 MHz scope. Well, 50 MHz is only the 5th overtone of 10 MHz, so my view on the output is a bit myopic.

So I'm going to order a set of boards to try with footprints for a 5 pole filter instead of a 3 pole filter. That ought to improve the filter's performance a bit (hopefully enough).