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

Hate to break it to ya, but...
A) mine looks nothing like that
B) I sourced my own parts from 7 different vendors. 
C) Mine drives more current and more voltage than Nelson's. 
D) mine has no current limiting like Nelson's does. 
Bottom line: I built a significantly more muscular F5 than Nelson did. Nobody is selling an F5 kit in a box. The only prefabbed parts you can get are the chassis and the boards. For everything else you're scouring DigiKey, Mouser, Antek, or whoever else has what you need because nobody even sells all the parts to build one. Go to DigiKey and tell me what are the best resistors for R9 through R12. 
I'm not at all saying I could design the thing by a long shot. In fact, as boxes of parts started showing up, I started wondering if I could do this. It took about $1000 to buy everything and I was really worried I'd reduce it to garbage at the flip of a switch. The opposite actually happened. The first time I flipped it on as a complete unit it did exactly nothing. Zero voltage or current at the outputs no matter how I turned the pots. It just sat there cold and dead with glowing LEDs. I was bummed. I gave up and slept on it. The next morning I realized my mistake. I forgot to recalculate the resistor values in the current limiting circuit. Mine we're too low for the higher rail voltage I'm running and the higher voltage at the BJTs bases was driving them to full conduction. It was a stupid mistake anybody with experience wouldn't have made. So I pulled the BJTs. Turned it on. Hooked up the DMM, and I got current! 
The F5 is not the easiest amp to build. The Zen Variations are. The F5 is odd in that he's taking advantage of using the transistors in different modes simultaneously. It's a much less straightforward design than the F4 or F6. There's a TON of stuff to learn about what every single resistor does. 
I really wish you'd just build something. Go through the process. Realize how much you don't know and learn it like I did. Go build an Aleph L. Get your fingers on the parts and really know what everything does. 

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.