Graduating to separates need help


Graduating to separates need help
I currently have a Marantz SR 18 receiver  that was feeding and Adcom GFA 555 that has been modified. my SR 18 had a memory issue and the local shop messed up the micro processor and I can’t set my speakers to large now in the menu
I also have a Denon AVR 5600 receiver which I’m using now and there is no comparison in the sound the Marantz destroys the Denon

I don’t know enough about preamplifiers to buy a good used one I need something with good DAC  
the DAC chips in the Marantz sounded phenomenal. should I just buy another SR 18 and feed it to a better amplifier or should I jump to a parasound preamp P6 and buy a better amplifier I was looking at some Bryston amplifiers which I really like but my budget is not big I want to keep my budget around 2000.  I don’t mind used.  I just want better sound than the sr18 adcom setup.  Any suggestions 
rab0829