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
.....Foegotcto mention that I’m not sure about the impedance  matching for that? Someone here more knowledgeable could answer to that.
No comments on the Denon AVR-5805 with 32 burr Brown DACs   And Audyssey     32 bit processors 
I keep reading on forums that in its price range there are no pre pro combos that can compete.   
I can still use my adcom.  
Or use extra money to upgrade to a better amp.   Adcom sounds ok to me more powerful but the old marantz sr18 amp sounded warmer.   And the denon avr5600 is actually pretty strong but the preamp section of the old denon is grainy harsh and lacks detail 
The world is your oyster right now, many good options. The only advice I would give is seperate dac for sure! The tech is moving fast. For audio anyway. Good luck!
Seems the separates patrons have been silenced.  Lol.  

Really need to test side by side. No way to remember one from another.   
With that budget, you need to give a call to Odyssey Audio. You could likely get a very good package deal for not much over $2K.