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

No preamp is still a preamp. It’s just built into the device.  

Generally a good stand alone preamp fed by the line level output of your source will be better than an amp fed directly by the volume controlled output of your source.

FWIW, devices that only include line level outputs are less complicated, have better interior layouts, and sound better than “do it all” devices that include direct volume control capability.  

It’s really pretty much about the two methods of volume control.  My ET7 and my SP20 pretty much whoop the volume controls that are available to me using any of my $2k level sources.  I’m better off feeding undegraded line level into either SP20 or ET7.  Same was true with ET3SE or PV8 or PV11.  

I am using simple terms to keep from obfuscating the issue in play.  

I’m happy to agree to disagree.  

But experience tells me that, as seems to be true of most forum dwellers, I should wait with bated breath for you to have the last word.  

Go go for it.