Do you have any other Topre boards to compare the FC660's dome weight to? I have been through several and the variance is pretty remarkable. I'm looking for one that feels like a 45g Realforce or Novatouch, i.e., very light.
Here's my assessment with my Topre boards in terms of lightest to heaviest feeling:
Realforce 104UG HiPro < Realforce 86U (arranged domes to uniform 45g) < Novatouch w/ HHKB sliders < Printed Leopold FC660C < Realforce 104UB-DK < Topre Type Heaven < Realforce 87U 55g < Realforce 87U 55g dental band silenced
However, aside from my silenced 87U, nothing has beaten my 86U in terms of silence. The 86U doesn't have very harsh thocks, however, if you eliminate it from my list, the printed FC660C is quite a contender for being light and moderately quiet. Unlubed, the Novatouch and Realforce 104UB-DK are louder than my unlubed printed FC660C. Lubing and adding shelf liner both inside and under the casing of the FC660C has made it very light and quiet, but still not to the degree of my 86U, which is unlubed.