A Tube amp! Adcom is pretty benign gear, they add very little coloration to the music. I would start with amp (s) then look at devices. I use to own a Adcom 750 GFP pre-amp along with the GFA 555 amps. The soundstage was very wide but it missed the depth and instrument separation.
DAC upgrade or Amplifier
Which has more influence over the soundstage - the DAC or the amp? My sound stage is high and wide, but recordings sound 3 dimensional only rarely. My equipment is at best mid fi - Adcom 5500 amp and Cambridge audio DAC magic. Listening primarily to Tidal at their highest level available. The rest of the system if Adcom Preamp 750 (the Nelson Pass design), Tyler Acoustic speakers, tributary cables throughout. Any advice or suggestions for upgrades for improving the "depth" of the soundstage is appreciated (let's say that an upgrade for less than $1500 is in the budget, and I'm comfortable buying used from reputable sources). Thanks!