Preamp Choice... a hard call for me.

A couple of DAC's you could consider. At the Budget end the Topping D-50 DAC at around $249. At the upper end the RME ADI-2 DAC. Also has balanced out and has an excellent headphone output. Supports both standard over ear and IEM head phones. Made by RMe a German Pro audio company. $1099

I have a weird but perhaps useful suggestion. Just  find out what can REALLY can outperform your preamp (W/O any DAC) regardless of the cost, just so you know what is possible. Then ask yourself what your speakers are capable of reproducing. Your amplifier choice will also be critical. Otherwise you may, as I found out- trying to get a source and a preamp (and last of all some good cables) to "come up to" the level of resolution of your speakers. That is the beauty (and unfortunately the expense) of separates. I went through too many upgrades (although I had some fun doing it) to get my speakers to sound their best. But my ears led the way- sometimes I heard good things happening and sometimes I really didn't. 

Funny that this started with a question about a preamp and everybody is now talking about DACs!

We doubt there is anything wrong with the Denon.  We used a 2900 for years and it performed very well, and will certainly be a stout transport if you decide to get a DAC in the future.

If you want, let us know where you are  located and maybe we are close enough to bring you a lightly used Rhumba preamp for an audition in your system.  We can sell it for $1900.  There is no DAC in it but we think you'll find that the improvement of the quality in the preamp will be so significant that you will be "done".  Retail price was $3000, this unit was sent to us for upgrades and the client unfortunately passed away. We have completed the upgrades and volunteered to sell it for his widow, the full amount will go to her.