Amplifiers don't have much effect on the sound, so I think you are looking at wrong direction here. Generally electronics have 5% effect on sound and the speakers 95%. Speaker positioning and room acoustics have quite big impact to sound as well, so I would check this area first. You have good setup there, but changing amp/pre wont get you far from there. If you can't get it sound good in your room it is time to start listening to more speakers and find a speaker you like.