Both is possible. You can feed the Preamp with external Power or a bias T via the Koax line. Please note that a duplexer has a DC pass only at VHF. For UHF you have to feed the power direct to the amp. Buy the way, I'm using a Duplexer as well.
I don't want to practice some advertisement, but these company produce extremely good LNA as well.
Maybe you can check this as well.
vy 73 Alex