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

It's the riddle for which there is no answer, the corollary to "not all things that can be measured count and not all things that count can be measured". System dependent, but in many cases, a great preamp creates the "jump factor" that turns a boring system into an attention grabber. That's a fact. Debate it all you want. 

System dependent, but in many cases, a great preamp creates the "jump factor" that turns a boring system into an attention grabber.

+1 @fsonicsmith 

"System dependent, but in many cases, a great preamp creates the "jump factor" that turns a boring system into an attention grabber".

True it can wake up a boring system ,but it can also make a system too alive and strident

I’ve never tried the Oppo direct but I find it very digital sounding (in a bad way) and that is through a VAC preamp.  So, I don’t think any preamp is going to help.  If you like the sound direct, stick with what you have.
 I would suggest a better player or add an outboard dac that sounds better and reinsert your preamp.  That said, it’s your ears not mine that count.

Crap! I must have hearing problems. I thought the darned Oppo sounded pretty good!