You're not hearing what your amp is capable of. I agree with others that the preamp has more to do with the sound of a system over an amp; they're both important. I've heard ok amps sound very good with a great preamp, but not a great amp sound great with an ok preamp. Try the Rogue pre... can't go wrong with that brand.
Is upgrading pre worth it?
I Own a Denon 7.2 receiver. I am using it for a pre. I have an st100 Rogue amp. What difference would a nice pre make? for 1500 used Rogue pre would I get 1.5k worth of sound.