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

To be fair, there is no reason to use a preamp if you don’t need the gain (e.g. you are listening to vinyl).  It’s not going to sound better, which is what the OP was about.

“No preamp” is a straight wire. It will add no noise, no distortion, and in 99.9% of cases no frequency response anomalies.

There is not a preamp made that has the same characterics. Thus, there is no amount of money that can be spent on a preamp that will sound better that a straight wire.

There are reasons someone might prefer a preamp to a plain interconnect. These include gain, impedance matching, and pleasurable distortion.

Personally, I have retired my preamps (I have three to choose from) and have my DAC connected directly to my amplifier. It is better. 

Replace your amplifiers with a wire and let me know how that works out for you.

Since I listen to speakers, my DAC doesn’t have enough current output to make much noise, so I think I’ll keep my amplifiers (my speakers are bi-amped). My preamp wasn’t doing anything useful, as the DAC has enough output to drive the amps and the speakers to 100+ dB peaks.  The preamp was only adding noise, distortion, and frequency changes. 

There's no technical reason to conclude getting gain in a pre-amp stage won't sound better.

Sure there is. Gain means an active circuit. And all active circuits add noise, distortion, phase and frequency distortions. If you don’t need the gain from the preamp, you can avoid these problems by eliminating the preamp. 

“No preamp” is a straight wire. It will add no noise, no distortion, and in 99.9% of cases no frequency response anomalies.

There is not a preamp made that has the same characterics. Thus, there is no amount of money that can be spent on a preamp that will sound better that a straight wire.

It is now easy to see that the above post is false. Its based on the idea that cables have no artifacts, also leaves out the effects of capacitance and the like that are inherent in cables and makes the false assumption that a volume control is somehow the same as a straight wire, which it certainly is not! As a result, you can actually have an active line section with wider bandwidth that can indeed sound better than a passive and not due to ’pleasant distortions’ either (since any properly designed active line stage will have vanishingly low distortion), but instead due to the additional transparency offered by cable control. The distortions created by amplifiers are far more significant!

It’s always nice to have something to argue against, but in my case, you are setting up a straw man consisting of a passive volume control and a high impedance output to the next stage. 
My volume control is the internal volume in the JRiver Media Center 64 bit DSP. My DAC has a volume control, but I keep it at 0 dB attenuation. The DAC has a low impedance balanced XLR output that goes directly into my amplifier. So while you can make good arguments against passive volume controls and uncontrolled cable interactions, these are not a factor in my system. Thus, in my system, my statement above that there is no amount of money that can be spent on a preamp that will sound better that a straight wire, is a perfectly true statement.
 I also think that the distortions caused by active circuitry are far more likely to cause audible problems than a passive volume control hooked up to a patch cord. Though that isn’t an argument that can be settled without specifics which are not available here. But I feel that I am on pretty solid ground in saying that anyone hooking their moderately well-engineered DAC up to their amplifier without any passive components between the two will get better sound than they would get by adding an active (or passive) preamp to the mix. This will be the case for most people, and was the scenario painted by the original post.

So while you can make good arguments against passive volume controls and uncontrolled cable interactions, these are not a factor in my system. Thus, in my system, my statement above that there is no amount of money that can be spent on a preamp that will sound better that a straight wire, is a perfectly true statement.

@phomchick I agree on the first sentence 100%. But not on the last; most high end audio manufacturers don’t support the balanced standard even though XLR connections are used (and to be perfectly clear, I’m not sure whether that includes Oppo). So if you were to add a buffer or active line stage that *does* support the balanced standard, you *may* find that it is an improvement.

"6 XLR Balanced Audio Output

"XLR balanced connection offers better common - mode noise rejection and improved signal quality over the RCA connection. If your preamplifier, power amplifier, or active speakers offer XLR balanced inputs, you may use a pair of XLR audio cables to connect the ******** DAC’s XLR Balanced Audio Output to the XLR input of your device."

@atmasphere are you now willing to say that both my statements are 100% true? :-)

In summary: the OP removes his  Audible Illusions L1 preamp and drives his amplifier directly from his Oppo BD105. He thinks it sounds better. He asks: how much money do I need to spend to get a preamp that will sound better than this straight wire?

Answers (boiled down from this long and contentious thread):

1. It is not possible to spend enough money to improve the sound beyond what you hear with no preamp. Preamps add some noise and distortion. Wires don't. Sell your preamp and take a trip to Paris in the spring.

2. Some people like the euphonious distortion that some electronic components add to the signal. Often, the more money they spend on the component, the better they think it sounds. Spend about as much as you might budget for a new car on your new preamp, and you are likely to feel it sounds better than no preamp.

3. Although not presented as a possibility by the OP, a class of responses caution against adding a passive preamp in place of no preamp. These arguments talk about the impedance of the upstream and downstream devices and should fall into the category of why don't you try it and see what you think? Sadly, this advice seems instead to be: don't ever try this, it's theoretically unsound and your house might burn down.

Did I miss anything? 

Well I just bought a used Ayre K-5x pre. Any opinions?

Only a question: if you were happy with no preamp, why?