Can it get better?

Seeking the advice of folks who have experienced significant gains when moving from an integrated amp to separate pre and power. I realize there are several threads on the topic but specifically, I’d be concerned about a lateral move in terms of sound quality while dishing out close to 2x what I purchased my integrated for. For example, my system consists of a Pass Labs Int-25 driving Harbeth Shl5+ 40th anniversaries. It’s a combination that gives coherent and smooth sound yet is quite resolving. 
Why change? For one, I would like to experiment with different power for the Harbeths and am intrigued by some of the class d options out there. The Pass was about 5k and some of the pre and power combinations I’m considering would push the cost north of 10k. There are a number of well designed preamps out there in the 5k range that I could see serving me for many years. Assuming good synergy with the power amp, is there good reason to believe that separates would sound significantly better than the little Pass which has system synergy built in? In what way? Or would I have to venture into a higher tier in terms of cost?

thanks for the advice

 Brian