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

You need to back off.

Sounds like a threat there sunshine. You are an agitator, as many on Audiogon have said, stop drinking or whatever your doing. 
Like I said.
That’s the second time you’ve put **** on Nelson Pass, who do you think you are? better???, your not even his hang nail.

As others have said, you built a F5 from "paint by numbers" "kit" that anyone that can hold a soldering iron can put together.
http://www.6moons.com/audioreviews/firstwatt9/1.jpg
And you think you know better than Nelson Pass, who gave that very amp to the diy’ers who can solder to make.

Cheers George

I'm not at all saying I could design the thing by a long shot.

That's about the most honest thing you've said, enough putting**** on what Nelson Pass says about passives.

Cheers George

@georgehifi , kosst claims his system sounds almost the same, with and without the preamp section of his ancient Marantz receiver, when he experimented with the bypassing of the Marantz. This should tell you something about him. Let him be, let him rant, he is an annoyance to many people. He just needs to be right, that is all. No wonder he is afraid to try an upgraded power cord, as he fears he will not hear a difference, let alone an improvement. I must admit that I do enjoy the 2 of you going at it, as it is very comedic. I wake up every day and wonder who is kosst fighting with today, and yes, there he is. Look at the recent Focal thread. Hilarious. BTW, I am wondering if the F5 that kosst built would pass UL certification, which Nelson’s does.? Enjoy ! MrD.