I've said this before when the subject has come up on the Plomb thread on the vintage board, but I'll repeat it here. With so many credible available options from other words in our language to choose from - e.g., comb (oh), bomb (ah), or womb (oo), it would seem like it could be pronounced like any one of them, and hard to assert one of them over the other. It's very instructive, though, and, in my opinion, impossible to argue in good faith against the idea that a company who consciously incorporates plumb bob imagery into their trademark application, plumb bob imagery into their marketing, and plumb bob imagery into their very name sure seems to be practically begging you to pronounce it like "plumb".