AMP or Preamp

Don’t let anyone criticize you over your current components. Your receiver is certainly your weak spot and although there is nothing wrong with a Rotel amp, I have to believe the Wilsons would reward you for giving them better power. 

Several people have mentioned Pass, which would probably work well. If you like solid state, the Plinius or SimAudio are reasonably price brand to consider. Sim is a little darker while Plinius has a more tube like character. 

If you like the tube sound then then you may consider Audio Research. Wilson dealers often pair them together and it does sound very nice. AR is hybrid so they can better handle the challenging impedance curve and power requirements of Wilson better than full tube. 

I would seriously consider an integrated. Although separate amp/preamp would be ideal, you will pay more for the components plus you will need cables which adds to the cost. Improving components without using proper cables is somewhat of a waste. An integrated removes the need for the extra cables, so you can spend a little more on a better integrated and put some money towards power and speaker cables. 

Enjoy the journey and have fun!

check out the Aesthetix Mimas intergrated amp hybrid can add DAC Phono or high pass   
Great components 
and I second  Audio Connetion great bunch of guy and sell AESTHETIX

McIntosh C47 pre
McIntosh MC-312

ENJOY!!!

Definitely upgrade amplifier before anything else. You need a high current amplifier cos the Sasha’s impedance dips very low around 80 Hz.  I stand corrected on 80 Hz. 

Lots of good suggestions.  Going to take some time digging into all this.