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

+1 PHD.
As this tested McCormack shows, absolutely no need for an active preamp, when most sources are >2v or more out, and this McCormack only needs 828mV in for full output wattage. 

Stereophile: " Input impedance at 1kHz was a calculated 99.5k ohms, consistent with the manufacturer's specification of 100k ohms. The DNA-1's gain was very high, this gain ratio corresponds to a decibel gain of 32.4dB, implying an input sensitivity of 828mV for full 150w output."


Cheers George

I  did buy an axiom passive preamp. Although it is very good it lacks in bass and warmth for me.  I'm using the audible Illusions again.

Although it is very good it lacks in bass and warmth for me. I’m using the audible Illusions again.

That is the most common experience we hear about passives.

honashagen OP
I did buy an axiom passive preamp. Although it is very good it lacks in bass and warmth for me. I’m using the audible Illusions again.

The only reason for lack of bass, which would also rob warmth, is if the source’s output is "capacitor coupled", and that capacitor is too small.

It’s a simple fix to double the uF (microfarad) of the capacitor and at the same time use a better "quality" one, as most skimp here with average quality industry ones.
  
Myself, all my sources, and everything else is direct coupled, which has the best bass, and no "sound of a capacitor" in the signal path.

Cheers George

If you think your 105 sounds good now, check out replacing the incoming IEC and wire (from ebay (https://www.ebay.com/itm/Oppo-95-105-105D-205-Mod-Kit-Mundorf-Cable-Bypass-Voltage-Selector-Furutech... )

and the Linear Power Module (mine was only $113, this might be an upgrade)  https://www.ebay.com/itm/Digital-version-Linear-Power-supply-OPPO-player-For-BDP-103-103D-93-PSU-Mod...

I am using a Parasound P5 in XLR . I never thought to bypass it post 105 mods, but now...