dastro
Your Threshold Threshold S500/II (great amp BTW I had one) only needs around 1.2v input to give full 250w output, so a low gain preamp would be best to have so you can have a good range of rotation on the volume control, as most sources are over >2v output.
And it has a 75kohm input impedance, which would be a great match for a passive preamp. The way I used it.
I would say being new school this maybe better and it’s new with warranty and it has remote, trial the Schiit Audio Freya (from the guy who designed Theta Audio’s best products) , as it can be a passive pre, active tube pre, or active solid state pre. So you can suit your moods. All with low gain so you’ll have plenty of range control with your volume control.
Plus it has balanced or rca input and outputs and comes with a extremely high quality 128 position relay based volume control, and it’s all remote controllable as well. For just $699 with a trial period, so you could send it back if you don’t like it.
http://schiit.com/products/freya
And they make a great phono stage as well mm/mc.
http://schiit.com/products/mani
Cheers George