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

@georgehifi 
You look at the schematic and draw conclusions that don't reflect the reality of the circuit you're looking at. And then you make stupendous claims about how magical it is. There's no way to avoid the fact that anywhere on that ladder you're getting a mix of direct signal and gain signal. I'm glad you like it. I'm sure it sounds awesome if you give it just the right home. 
 The service manual clearly says .2% with .02% typical. I'm sure it's virtually all second harmonic because that's what single ended MOSFETs make, and I'll bet it's also negative phase distortion, too, which sounds really neat. That's still a lot of distortion and color and limits your amp options. 
I'm not sure why you keep bashing the 15dB of gain my amp has. I'd much rather make voltage gain in a pre-amp than a power amp. It's just a better place to do it for all kinds of reasons. 

" Like so many things he's designed, it's a quirky component with a  narrow appeal ". I honestly cannot believe this has been said. NP has shared with the diy community most of his designs, which were all labors of love, not to make a dime. Give me, and everyone else a break !

@mrdecibel 

Are you crazy? The man himself makes it clear that the FW amps are weird little amps with very narrow appeal and application. 

Maximum Gain 10 dB
Freq. Response +0, -1 dB @ 10 Hz and 100 kHz
Distortion < .1 % THD

Hm. We can do that with 1Hz to 400Khz, distortion at 0.01%. And that's with no feedback. And all tube, balanced.