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

The best way to me, because it uses Sabre balanced + and  - dac chip outputs, is to use the XLR output on the Oppo direct into a true XLR input poweramp. And use the Oppo’s volume control, hopefully it’s used at 75% or higher so there’s no risk of "bit stripping" This should be the ultimate in transparency and dynamics.

Cheers George

Can we really expect a simple pre amp to improve on the mastering engineers ear and gear? If so just put a bunch of your favorite preamps in series and the sound would just keep getting better!

No sir. And while I haven't read ALL the post on this long thread, everyone, even the arguments, are with the goal of transparency. the LEAST amount of change to the signal. 

If you are happy with no pre right now, that's great. General opinion here seems to think it's the best way to go. You could even take the money you saved and spend it on 5k of interconnect and REALLY have the most transparent set-up. 

There's lots of options.

I just closed a deal for a Hatton passive XLR, so will see whether it improves on no preamp. If no glitches, I should have it by friday

Tweak: sounds like a worthwhile experiment. 

Questions might be, is there a difference having the extra jacks and cables? Is the volume attenuation by the new per better than the digital of the Oppo? 

Is the Hatton buffered or is it just pure switching?