Also, and I'm not the expert on this, I'm just going from memory . . .
There are two reasons Hammonds sound different depending on age. One is that, as the years progressed, the manufacturing changed slightly. The Hammonds from the 70s, when they were about to be discountinued, they were using basically any old leftover parts and the quality wasnt as good. In the very oldest Hammonds, they didnt have the same technology as was available later as far as types of capacitors and all that.
The second reason is, resistors and capacitors tend to drift with age. Usually they drift to sound muddier, but also sweeter. That 1957 B3 probably sounded a lot brighter when it came off the line as it does now! The solution to this problem, and this is what the big organ stores and techs do when they resell an organ (at least some do, they should) is REBUILD the preamp. You take out all the original resistors and capacitors and put in brand new ones. Someone who knows how to solder can do this themselves, I did this to one of my Leslie amps and you wouldnt BELIEVE how it sounds. This also makes it less likely the organ will have problems down the line. But the older the organ, the more the componants will have drifted, and the muddier, less loud, etc it will sound. Especially the real older Hammonds, like I said, they didnt have the same technology to make capacitors and such so they were a lot more prone to drifting.
The combination of these two factors, I've always been told the best Hammonds are the ones from the 60s. They're from before the production started to go downhill, but not so old that they're way off. Whereas a BC or BV or any organ from before they made B3s, if it hasnt been rebuilt, may sound quite muddy! That having been said, any Hammond can be rebuilt to sound like new.
I'm mainly talking about rebuilding the preamp, and possibly also the Leslie amp. Many sites offer capacitor kits to rebuild the tone generator filter network, now from what I've heard, as well as my experience, this usually isnt necessary. Dont worry about that for now.