The Standing Room has some advice on choosing Lawrence Tibbett transfers, but he doesn't give a lot of detail about why:
As for the Tibbetts, if you ever find yourself in the unlikely position of having to choose between the RCA Victor Vocal Series album and the Nimbus Prima Voce "Tibbett in Opera", go with the RCA. There's about 85% overlap -- not just repertoire, actually the same recordings -- and the RCA remastering (produced by John Pfeiffer) is just better.
It's probably not that unlikely a position to be in (and there are also good Tibbett transfers on Delos). Nimbus's transfers are easy to find and they're usually inexpensive. I remember seeing them a decade ago for $12 when other historic-singer CDs were going for $15, or even $18, for Romophone.
But The Standing Room is right: avoid those Nimbus CDs like the plague. Their transfer technique is bizarre, to start with. They hang a microphone in a room, then play the 78 on a gramophone fitted with a giant horn. (Everyone else uses modern equipment, the more the merrier, because well-regulated modern styluses do less damage to the old recordings than period equipment.) The resulting transfer is awash in reverberation. At least it's not artificial, but the singers' voices are dulled and obscured by the sheer quantity of reverb.
I must here confess that when I started collecting, I thought the Nimbus transfers were great. That was because the reverb does successfully mute the surface noise found on a majority of old recordings - and it's true that, say, a Pathé hill-and-dale 78 is pretty darned noisy. Nimbus also has decent documentation, nicely-produced booklets, and an imaginative touch in choosing tracks for a compilation. But eventually I started hearing better transfers, and eventually I learned to listen past and through whatever surface noise other transfers don't conceal. Eventually I bought the noisiest records of them all, the Mapleson cylinders, and after that nothing seemed too noisy. Over time, I've replaced most or all of my Nimbus transfers.
If you're thinking of buying historical recordings, you have plenty of choices. There are good transfers of vocal and instrumental music on Naxos, Romophone, Marston, Pearl, and a number of other labels. Read rec.music.classical.recordings and other forums where knowledgable people post, and see what they think of specific transfers - engineers vary in their ability and style, and it won't take too long for you to find out who is good. But leave those Nimbus transfers at the store.