Tube Preamps.....

Used:
Belles
Cary
Conrad j

I think you'll find your options very limited if you want a tube pre with a built in DAC. You need to decide how you want to allocate your $3K between the two components and then search for threads which discuss these pieces individually. Lots of tube preamp threads out there. 

$3000 wouldn’t be split. $3000 will be preamp alone. Been reading a lot. Thought I would toss the question out with my current components. Could stretch it to $3500. 

Pretty simple choice.

1. Primaluna Prologue prem tube pre amp.
2. Denafrips Ares DAC.

The two items will come in just under your budget. The Ares is a giant killer of a DAC.

With the Adcom amp, I think you’ll enjoy the tube preamps that lean more on the "tubey" side of neutral. I have an Adcom GFA-555 that has been paired with my GFP-750 for many years and I recently bought an Aric Audio all-tube preamp for another system and I really like how it sounds with the Adcom amp. It gives it a nice lush and engaging tone, particularly in the midrange, that the Adcom preamp lacks. Some may say its not neutral but it just rounds off the edges without losing much detail. I think you’ll be delighted when you put a tube preamp in front go your Adcom and for $3K, especially used, you can get something real good.