While I own an integrated and many separates, my comments come from fifty years of pursuing high end audio and owning many pieces of Pass equipment.
Your concern is well founded. Your integrated has the advantage of single design, savings in costs by one case. Long ago integrates we’re not very good. Now they are. So, it would be easy for the switch to be largely lateral. However this is treating preamps and amps as a commodity… all are exactly the same just better with cost… this is not true. The outcome will completely depends on exactly what you buy and how big the change in character there is with the new equipment. I would say that careful selection you can get very significantly improved sound… but you must choose wisely.
I was surprised to see your amp does not have preamp outputs. The traditional way to upgrade… power amp only. I am disappointed in Pass not to have provided preamp outputs.
A couple thoughts.
Long termed significant improvements will require separates… if you want to craft and customize the sound… this is the path forward.
Honestly this is where a dealer can be invaluable. At some point not trying to save a few bucks off of audio equipment by buying on-line becomes very well worth it. Also, if you are going to $10K (I would recommend) a dealer. A good one will figure out what changes you would like to hear, recommend possible choices, let you test the equipment at home and give you a trade in on your Pass integrated. Yes, of course he would most likely recommend what he sells… so research first to see what he sells… maybe target two or three places. Worth a long trip if you are in the sticks. Establish relationship with the owner in advance and make an appointment.