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

I should point out that this idea of taste applies only when we're talking about reasonably well matched impedances. If you have a low impedance amplifier input a preamp may indeed be needed.

However given the modern high-current op amps in most DAC's, most can drive 600 Ohms. Easy peasy. 

Best,

E

Rather than spending a lot of money for a better preamp, here’s a much less expensive possibility to consider, that could very conceivably result in a significant improvement:

I presume that all of the connections you have been using between the components you mentioned are unbalanced. Although I’m not certain, I suspect that as in the case of several other Oppo players the balanced analog outputs of the BDP-105 are driven via a signal path that is fully differential from the output of the D/A converter chips to the XLR connectors. And it has been reported in various posts here that some users of Oppo players have found the sonics of the balanced outputs to be superior to the sonics of the unbalanced outputs.

According to this review from the year 2000 Steve McCormack offered, and perhaps still offers, a mod to the DNA-1 which provides it with balanced inputs, implemented with a Jensen transformer. The cost of the mod in 2000 was indicated to be $450. And, alternatively, a Jensen transformer that could be used externally to convert balanced signal pairs for both channels to unbalanced, such as the model PI-2XR listed here (select RCA output connectors under "Pricing") can be had for ~$250.

If your DNA-1 doesn’t already have that mod you may want to consider either contacting Steve to see if it can be incorporated, or separately purchasing a Jensen transformer that would provide XLR inputs and RCA outputs.

Regards,
-- Al

No balanced inputs on my DNA1.

That Audible Illusions L1 is an excellent preamp.  You have arrived.

I don’t think you can look at it as a “level of quality”... instead it is more of a trial and error. I had an oppo 103 playing directly into my amp... it sounded good. I added a budget ( under $1k preamp) and it made a big improvement. Then I tried a nuprime dac9 as a preamp and it was a step backward. 

Bottom line, since lots of new equipment is returnable... and/or lots of preowned is resellable at virtually no loss... you have to try a couple preamps and see where it takes you.