Tube Preamps

I was wondering opinions on whether or not a tube preamp might fix or reduce my overly bright sounding system. I am currently sourced by a Rotel RCD_1520 with Rotel RB-1582 2x200 watt amp, Rotel RC-1580 preamp with B&W CM7 speakers. I know the speakers are a tad bright, but when I disconnect the Rotel amp/pre and reconnect my old NAD c317 integrated, it sound great. I like the warm sound and deep bass of the NAD. Isn't the amplifier just amplifying what it receives from the preamp? Is it possible for an amp to be bright?