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

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!

@honashagen 
you should consider any specific recommendation of equipment as anecdotal as it may not sound nearly as impressive in your system. 

Although unlikely, it’s absolutely possible that a $3k tube preamp won’t impress you as much as a $1k solid state preamp. 

Best to stick your toe in the water, buy something returnable and find out what works for you in your system in your room with your music.... all while realizing it may take a few tweaks to get it right.