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

I missed if those are being used going direct with the Oppo.

It is one case where going direct to amp with the balanced output betters the rca because the Oppo use a balanced output dacs in the Sabre which has + and - outputs in each channel, so it's naturally better to use it's xlr output than it's re-configured xlr to rca.

Cheers George 

Re: balanced vs rca on the Oppo: certainly, I might expect it to be better, but I mention it because it's another variable. AI pre's are single ended. 

If I understand correctly, the amp is a McCormack? I don't remember those as being xlr inputs, but I could be wrong. Maybe it was an option? 

I just recently got a UDP-205, and it is going into an Audible Illusions M3a. I haven't been able to compare the rca vs balanced, I don't have the stuff. What I CAN say is, to spite having the "best dacs available" or at least the best for Oppo, the old Theta Prime 2 seems to be a batter dac. 

I mention this, because as far as "upgrading", the rca vs XLR is right there in the picture, ain't it? A balanced pre makes sense if the other components are balanced, even better yet, TRULY balanced, but it doesn't make sense to ugrade to balanced to take advantage of the Oppo's balanced outputs when a better dac could be had for less money.

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.