In theory the medium mass Rega should work fine - I've used the 103R with my medium mass (~11g) Graham 1.5T and it worked beautifully. Sounded fabulously involving and was (surprisingly) one of the best tracking MC's I've used. When measured with the HFN&RR test disc, both vertical and horizontal resonances were around an ideal 11Hz.
I think the problems some people have with medium mass arms is dealing with the resonances of the 103's plastic body - not incompatibility with the primary resonance of the cantilever system. The heavier arms might be damping the actual body resonances better - on some tables. So technically medium mass arms work just fine, but IMO would need to be a quality arm/table to get best results. I think there is a lot of anecdotal evidence from 103 users to support this.
Also - with reference to the reply from Rodman99999 above - the Denon test graph that came with my 103R was done at 47Kohms(!), but I would not recommend using this load for listening. 100ohms sounds best in my setup.
I think the problems some people have with medium mass arms is dealing with the resonances of the 103's plastic body - not incompatibility with the primary resonance of the cantilever system. The heavier arms might be damping the actual body resonances better - on some tables. So technically medium mass arms work just fine, but IMO would need to be a quality arm/table to get best results. I think there is a lot of anecdotal evidence from 103 users to support this.
Also - with reference to the reply from Rodman99999 above - the Denon test graph that came with my 103R was done at 47Kohms(!), but I would not recommend using this load for listening. 100ohms sounds best in my setup.