How much do I need to spend to get a preamp that sounds better than no preamp?

You should have ditched the Oppo or added a decent dac.  

honashagen OP
How much do I need to spend to get a preamp that sounds better than no preamp?

$49 closest thing to going direct with a only potentiometer in the way.
http://www.schiit.com/products/sys

Or if you like to change the sound 3 different ways $699.
http://www.schiit.com/products/freya

Cheers George

honashhagen,
Many years ago Adcom had a pass thru line stage, touted as no coloration. I tried it with many amps. It did not seem to impart any color to the sound, yet something imho was missing. Well, not just imho... something was missing. The dynamics seemed compressed, and while there seemed to be minimal if any serious coloration to the sound (the bane of some preamps), there was also a lack of texture, space and fine detail. You know how when everything comes to life in music, the actual physical distance between the guitar and drums, the bass player... that was missing when compared to a good preamp. 
In fact the last comparison was putting a Gryphon Preamp, (very, very expensive) with any and all of the amps, all areas of aforementioned subtleties came to life. The preamp seemed to take the amp, even an Adcom 60wpc, to another level. It was a great thought experiment that I still remember more than 20 years hence.
So, I recommend a good preamp at whatever price you can afford... BUT experiment... you won't regret it.
Best,
Larry

Hi Ben, from the responses, it seems the sound of a preamp is quite dependent on the system and personal preference. I tried something yesterday that I have tried at least 20 times previously, with the same result. I know, the definition of insanity, right? I replaced my unity gain buffered preamp with a Goldpoint passive. At first, I really enjoyed the quiet background, nice tone, clarity, and purity, but after about 5 - 6 songs, I had the familiar sense that something was missing. I reinstalled my active/buffered preamp and.....Technicolor! The preamp had the same attributes as the passive but added tremendous body, dimensionality, and tonal density that made the music sound more "real."
Since you asked about what level of quality it takes to improve the sound over a passive, here are my thoughts.

• The passives I own, have owned, or have tried on an extended basis include, Goldpoint balanced passive, Endler Attenuators balanced, Adcom GFA-750 (mentioned in a previous post), and a Bent Tap type preamp using autoformers - Acoustic Imagery’s JaySho.
• My active/buffered preamp discussed above is a custom pre from Steve McCormack of SMc Audio, based on the TLC-1 platform modified with many improvements from Steve’s VRE-1. While it does not provide gain (unity gain) the signal is buffered and the preamp uses the VRE-1 choke power supply, a custom attenuator with AN Tantalum resistors, special Lundahl transformers (for a balanced signal), and more.
  
• I have tried more than 20 (tubed and SS) preamps in my system and the SMc preamp is the best I have heard. Several of the preamps had a list price north of $10K. My second and third preferences were Tom Evan’s Vibe and Pulse and Lamm’s LL2 Deluxe, both single-ended units.

Like you, I also own (but don’t currently use) one of Steve’s upgraded amplifiers that you can see on my system page. If you can stand not having remote control, I would recommend discussing options with Steve. IMO, the preamp is really that good and would match well with your upgraded DNA-1. If desired, Steve can design the preamp with +6dB gain from the transformers.  You can mention my preamp, which is pictured on his website. He also has other options he has more recently developed.

Last night I think I talked myself into thinking my new pass preamp was sounding good. I was fooling myself. It still sounds too forward to me. I really want to like it.