Keep Vintage or sell for new

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. 

Thanks unsound and Bourne

Appreciate you taking the time

I think it's a great preamp...my mind wanders to what else might be out there...the Pass Labs and PSAUDIO are a chunk of change for me so your responses are quite helpful.

Stick with the FET 10. Just because something is new doesn't guarantee that you will like it better. A good case in point is the 65 year old Quad 57. Many Audiophiles and speaker manufactures still consider it the Holy Grail in speakers!