I now have some interesting comparative data.
I built a bridge from 4X 10KΩ resistors and drove the upper and lower arms of the bridge with ULDO at +26 dBu.
The center of the bridge was connected to the balanced input of the notch filter.
The post-filter gain is 40 dB with a CM rejection stage following the post-filter amp.
The CM voltage was +20 dBu; 7.75V RMS.
The test Yageo was value-matched within about 4 Ohms to the near-perfect value of the CMF55.
The lower two arms were 1984-era Dale RN55s that I had previously matched to 0.01% with a 4 wire Kelvin bridge.
In the following FFTS one or both of the upper bridge arms contained a test resistor.
Remember that the drive level is +26 dBu, attenuated by 6 dB in each arm, and that there is 40 dB gain.
A -60 dB indicated 1 kHz FFT level is actually about -120 dBu.
A 1Mpt flat-top FFT was averaged 10 times.
The first FFT is with 2X Yageo MFR-25 in the upper arms and 2X Dale RN55 in the lower:
As you can see in the above it appears "distortionless." If the distortion signatures of the upper arms' resistors match each other - and the lower two arms' signatures match there is cancellation. This doesn't mean any of the resistors are "perfect" it just means that the upper pair distort similarly to each other and the lower pair distort similarly to each other.
This FFT is with 4X 1984-era RN55s:
Again there is cancellation.
The following FFT is with 3X Dale RN-55 and a single Yageo MFR-25. One of the RN-55s is in the upper arm and we can now see a relative difference in the signature.
OK, so the Yageo distorts differently than the Dale RN-55s.
The final FFT may be the smoking gun. It has 3X Dale RN-55 and a single CMF55:
Relatively speaking the CMF55 has about 5 dB more HD3 and introduces HD2 compared to the Yageo. The insertion loss of the notch at HD2 is about 9 dB so the HD2 introduced is far worse than it appears.
This is a comparative test but may well explain the increase in HD3 when I use the CMF55 in the oscillator.
UPS arrives in a few minutes and I'll test the Holsworthy and Vishay MBB0207.