Hi Marc, I'm far from expert, but the higher the DB figure, the higher the gain - DBI is a gain figure over an Isotropic antenna, as far as I know that is an omni-directional ("theoretically" perfect) antenna, with no nulls.
Pre-amps require a power source, which is usually 12v DC, directly connected by a psu in the case of the JIM M-75, or with external devices, such as the Elad or Kuhne which are mast-head amps, which get their power supplied through a Bias-T unit. A Bias-T box has a connector from antenna, output to the scanner, RB, SBS or whatever, and a socket for the 12v supply to be input, the 12v does not go to the output side at all, but is "pushed" up the feed from the antenna where it is received by the pre-amp. All extra connections incl. pre-amps and bias-Ts create some losses, and will also create some "noise of their own. Pre-amps come into their own, if used at the antenna, to overcome the losses incurred by longer lengths of co-ax. Also, bear in mind, if you've got a crap signal in the first place, a pre-amp isn't the cure-all some think, get the antenna and downlead sorted, would be my advice in the first instance, otherwise the pre-amp may just make the "noise" louder.
I hope this is of some assistance!