Now, I'm confused.

I have an ADCOM, GFA-5500 and run it with a vintage Audible Illusions, Modules II, tubed, pre amp. The combination, for the $, presents a very nice - wide, deep and detailed - sound stage. One thing to note - the ADCOMs, like many other SS amps, need several hrs. of warm up to sound their best..Jim

+2 Ipretiring. Diffusers diffusers diffusers. 

+3 lpretiring
My soundstage became larger/bigger/deeper only after I installed room treatments, and I'm listening to SS, not tubes.
I'm using Primacoustic panels, purchased from Sweetwater Sound.
No vested interest, just a happy customer.
Read Jim Smith's book Get Better Sound.
Do what it says, before you spend any more $ on gear.
Your room will dictate 50% of what you hear.

I disagree that changing preamps will not give you are wider soundstage. 
Definitly try a tube preamp. I had owned Adcom 555 in the past. While they can be a very good sounding piece of equipment, they can be somewhat cold and clinical, especially pairing solid state with more solid state. I made a huge improvement in my soundstage by replacing my solid state preamp with a vintage tube unit. Namely a Dynaco Pas3. I have graduated to Audio Research tube preamps over the years, never going back to solid state preamps.
Good luck,
Mg16

How would that work? Better channel separation?