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.
Try 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 controlable 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
George,
Yea, I love that Threshold. It taught me a great lesson about amps. Before that I was using an Aragon 4004 Mk 2, which was a revelation compared to 2 ADCOM 555's bridged on my 802s. The Threshold grips and controls the woofers and completely changed the sound.
You are the second person to recommend the Schiit Freya. Mmmmm, I'll probably have to check one out.
Thanks for the help.