Will adding a preamp dramatically change my sound?

It depends entirely on the output capabilities of your source, and the sensitivity of your amp(s). If the source has the drive to achieve the SPLs you enjoy with your amp/speaker combo, you'll be happy. I thoroughly enjoyed the transparency I experienced with only a Placette Passive Linestage, and with MOST sources had all the dynamics, sound stage, extension, etc recorded on the disc. BUT: With those that were recorded at lower levels(some of my favorites) there was inadequate signal. Now I'm using a TacT pre that gives me just enough gain to overcome the differences in my discs, is still very transparent, and gives me a bunch of flexibility with regards to time domain and room tuning. Then again- There are a lot of people that enjoy the various things that pre-amps add to(or take away from) the music. Some mistake grain for detail, microphonics for warmth, etc. Then it becomes a matter of personal tastes as to what is "right" for them.

Yes, and it could be for the worse. Try before you buy. Sometimes less is more.

ET

I agree with the other posters that for the most part one cannot make a blanket statement that an active vs. passive vs. no preamp is always best. Unfotunately, like most all suggestions, there are just too many variables. One must try different approaches to determine what is best for each owner, considering the variables of different equipment, cables, room and musical preferences.