Keep Vintage or sell for new

Omg yes. I bought a brand new Threshold s500 when released in ~80. Then a T2 preamp … then a Pass x350 ten or twenty years later. I can’t begin to tell you how much better contemporary stuff sounds! 

Every ten years the sound was significantly improved. My old Threshold stuff sounded thin and transistor like. Each generation has been warmer, more realistic, greater current.

Pass labs (Pass being designer of Threshold) is a really good choice. But honestly I have to recommend considering  a tubed preamp. the preamp really is the soul of your system, and a tube preamp can add real magic to your system.
 

Audio Research, VAC, and Conrad Johnson make amazing preamps with great midrange bloom and rhythm and pace… very musical and you get the detail. But it is a mater of taste… but for me, once I put a pice of tube gear in my system… it would never be traded back to solid state. All my stuff is now tubed: Audio Research.

Hey Mission

Care to qualify your answer?

Keep the FET 10! Why spend thousands for a new preamp? Sonic differences between the FET 10 and today's gear are not enough to justice the cost! It was an excellent preamp when new and remains so today.

If the job of a line stage is just to transfer signal and offer control, I don't think you'll find much improvement over the Threshold fet 10 HL. In that role, it is one of the all-time classic neutral devices. And the basic high build quality with its corrosive resistant gold-plated circuit boards suggests that it could be an heirloom product. Which is not to suggest that a refresh wouldn't be a good idea. 

The Pass and BHK could offer convenience features such as remote control, and perhaps offer benefits via balanced connections. 

I doubt you'll hear an improvement that will warrant the extra expense.