Dear Rnadell: Yes, as Lewm posted that Ortofon M20FLSuper is a high compliance and very good one. Btw, I can remember any vintage MM/MI cartridge that was/is low compliance all the ones I know are high compliance, normally over 20cu.
Now, you know that Mr. Pritchard was the ADC founder/designer and that tonearm you own was designed by him, well after he left ADC he start Sonus company with new cartridge designs one of them was this one that's really good performer and that I have no doubt will be a very good match with the ADC tonearm ( the cartridge is very high compliance: 50cu, I think. ):
http://cgi.ebay.com/Sonus-Gold-Blue-Audiophile-Cartridge-NEW-FREE-SHIPPING-/220770169712?pt=LH_DefaultDomain_0&hash=item3366ed7370#ht_1227wt_1139
As Lewm point out too today MM/MI cartridges like the Ortofon 2M line are really good cartridges.
Rnadell, cartridge compliance always is important when we are trying to find out a good tonearm match but compliance ( along tonearm effective mass. ) only can tell us the cartridge/tonearm combination frequency of resonance but not its precise behavior or quality performance level.
IMHO the best we can do ( this is what I do all the time with success. ) is to mount the cartridge and see what happen.
I posted several times that in some of my cartridge/tonearm combinations the resonance frequency was really out of the " ideal " range: 4hz-6hz and I can't report any problem with its behavior or whole performance.
Now: could be that the same cartridge/tonearm combination can shows/showed better quality performance ( everything the same. ) if that resonant frequency been 10hz instead 5hz?, yes coulod be and that's why a removable headshell tonearm design is a desired characteristic because changing headshell weight we could approach that ideal resonant frequency range.
Rnadell, anyway our ears are the best judge about.
Regards and enjoy the music,
Raul.