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
Broke down and bought an AVR-5805 for 1300.  Sounds amazing. Super crisp and detailed.  However my old Marantz sr18 may have been better.   I just ordered an sr14ex.  I’m going to do a head to head
Update
Marantz Sr14ex trounces all others.  To be fair I’m going to order the mic to use audessey adjustments on denon.   Otherwise marantz wins

Finally got my goosebumps back 


I want to do a head-to-head comparison of the pioneer the Denon and the Marantz receivers. I want to use a digital optical output and be able to switch between the receivers and have the speaker level outputs going into another switching system that goes to one set of speakers anybody know what components I would need