Update: Regarding the standard DCS thin ABS doubleshots from SP, previously I had not installed the complete set on the NT. All the keys work fine with one exception: the Enter key. The Enter key clacks, just as all the keys did with the thick dye-sub PBT set.
At first, I thought it was the SP keycap, so I tried other Enter keys from SP with the same result (I have 2 sets; one black and one white, so that I can get a two-tone color scheme on 2 boards).
Next, I tried Enter keys from the Leopold PBT blank set, a vintage Dolch set, and a vintage Cherry set -- all with the same result: clack!
I have concluded that the problem lies with the hybrid Topre switch/stabilizer on the NT keyboard.
To alleviate the clack on the Enter key, I installed red 40A-L O-rings. I put one O-ring on each of the 3 stems in the Enter keycap. I also tried black soft landing pads, black 50A-R O-rings and blue 40A-R O-rings, as well as just one red O-ring on the center stem. I got the best result with 3 red O-rings; one on each of the 3 stems on the Enter keycap. This treatment results in the Enter key having about the same sound as the other keys; however, this makes the Enter key feel slightly mushier than other keys, and it is raised up higher than the other keys. These differences in the Enter key as a result of the O-rings do not seem to matter to me while typing, and on balance, I prefer having the improved sound for the Enter key.
The NT now seems more "Topre like", and subjectively at least, it now seems even quieter than my 55g RF 87ub. I am very pleased with the spacebar, which is quiet, and it responds evenly no matter where it is struck along its length.
The downside to the standard DCS thin doubleshot SP keycaps is that to me they look cheap. In particular, I have white alpha keys with black mods, and the black keys look like cheap pad-printed caps, despite the fact that they are doubleshots. Moreover, the keys have that clammy ABS feel rather than the slightly textured dry feel of PBT.