Help, which preamp to buy from my shortlist?

Pass 350.8 better than anything on your not so short list.

The 350.8 is an AMPLIFIER, not a preamp. And the XA series is better...

Sticking to the list, I have personal experience with

Bryston 0.5 - very good, unflappable, quiet. I wonder about buying it again, based upon some positive comments above.

Nakamichi CA5 first version - longest running preamp for me over the last 7 years, but now sold. I don't need to buy one back. I think of it with respect.

B&K Pro 10 MC - A useful bypass mode. Outboard power supply. Warmish.

All were well made, and offered flawless service, especially the last two after modern updates to return them to spec. All would likely hold close in value to whatever purchase price was agreed upon.

Despite my experience with these, I might like to try the Quicksilver from your list of preamps. I like tubed preamps for two of the three systems in the house. Would the quicksilver best either of them?

I may have to know this. I am also curious about the Audible Illusions. Let us know which direction you take!

I bought the Sonic Frontiers SLF1 last year and have greatly enjoyed it, fun rolling tubes and as every review says, the world's greatest remote!  I don't have experience with SQ with any other preamps on your list, but the SLF1 in my room doesn't sound too different than using my DAC's preamp capability, DAC being bel canto 3.7.   I really wanted multiple balanced inputs to easily A/B DACs and needed balanced output as this was the main reason for buying a pre, to move all gear except amp to a side wall, which meant a 15'+ run from rack to amp.

Best of luck, let us know what you pick!

Even though its not on your list, I recomend you seriously look at the PS Audio BHK Signature Preamplifier. Provides a lot of features and great sound.