I think I know exactly which older Potter bells you’re talking about. I have a Simplex-branded version, and I completely agree with your description; the gongs on those bells (at least the 6" models) resonate very poorly.
The new PDC models look like they’re rebranded Hong Chang bells, which I believe are the same bells that Mircom sells as the BL-6/10B. They certainly have nice-sounding gongs (and so do the Edwards MBs and their rebranded relatives)—they feel heftier than the gongs of the older Potter bells and reverberate nicely—although I find that nothing beats the sound of a 439D gong!