Advice / Counsel re: Upgading System

I would like to upgade my system to the next level, but don't have much direction as to what to do next. I live in a rural area with no high end audio dealers near. Current system is all Adcom; GFA5800 amp, GFP750 preamp, CGD750 CD and B&W 801 S2 Anniversary speakers. I love the sound, but want to go to the next level. Wondering about Levinson, Krell, AudioResearch, or ?? but would be acquiring pre-owned, not new. Any advice, things to consider, etc. appreciated. Thanks in advance!