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

I tried a Freya. Not for.me.

I have sometimes thought about constructing a version of NP’s B1 buffer

Nelson did make first the B1 buffer for the Lightspeed Lightspeed so it could drive my customers low input impedance First Watts and Pass Labs.
It wasn’t known as the B1 then, that came latter when he did a diy around it for the masses.
Here is a simplified circuit of his for the Lightspeed, before it was called the B1.
https://ibb.co/choBZS

Cheers George

@georgehifi 
Please believe me when I say I'm not trying to be a weenie here, but the B1 doesn't use complementary parts. It uses same type N-channel parts in a common drain mode biased by a common source mode part. Your diagram looks a lot like the voltage gain stage for the complementary voltage gain stage in the BA-3 BAGS, F5, and F7. Are you varying the voltages across the transistors to gain attenuation?

Most everyone knows that the McCormack amps have high enough input gain that allow passive preamps to drive this amp and as a result sound spectacular.

I am familiar with your Audible illusions L1 preamp and I liked it a great deal. It was said to be the same linestage used in their more expensive model the Audible Illusions 3 which may be debatable however It may not be to your liking nor the ideal companion for your McCormack DNA-1. Several years ago when I owned a DNA-1 with a Revision A the best sounding tube preamp to my ears was the VTL 2.5, and everybody else in the household that had a high appreciation for great sound agreed.