I believe in most cases you can connect a source directly to the amp, but it will need a volume controll unless you like the level it play's.
For years I'd had Been listening to my Denon 1500II CDP through a NAD 1600 Preamp/Tuner (a fine unit in it's own right) and an Audio Design PA100 Power amp. I was alway's surprised how good CD's sounded in a local audio dealer showroom. Better than my house by a lot. Of course they had better equipment.
One day I decided to connect the CDP's variable output directly to the power amps input. That day changed my whole approach to the world of audio. Suddenly things like sounstage/Imaging, Inner detail, Black backgrounds and coherency where all maknig sense now. It's not a matter of different, it's a matter of much better in every respect. It wouldn't go very loud though, but plenty loud enough to enjoy. That is what started my search for the most transparent equipment I could afford. My Energy Pro 22 speakers were already up to the task...who new, they never sounded this good! Too much bass for the 30' rec room in my new townhome though. Actually too little treble due to a carpeted floor and suspended fiberboard ceiling. So I replaced them with a pair of Reference 3A Speakers last week, that did the trick.
I finallly got what I wanted. However after reading Bluelobster's above post, I might set that CDP, Amp and my Energy speakers up in an unused Games room just for fun.