Keep Vintage or sell for new

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.

Your Threshold remains a great preamp with qualities that some new designs cannot dream of.  I remember the rave review at Stereophile by GH.

But reality tells that this is a 30 year old design, a long time for a preamp no matter how state of the art in its days.

Best is to compare it with your mentioned options side by side or take the leap of faith. Most likely it would prove a wise choice.