Whats a good matching preamp for Threshold S/300?

if you want a killer get a MFA lumi preamp--the combo will be sweet

A Threshold FET1, about 20 year old would be worth considering. Priced at around 2000$ when new, they usually go for up to 500-600$ and there is an auction on eBay right now for a nice MkII unit from Australia (at least judging by the pics). I own one that I bought 3 months ago for 220$ and I am delighted by the sound, includes an truly excellent adjustable phono stage (MM&MC). Add to that the fact it is a Nelson Pass design beautifully put together like some instrument-grade lab hardware, what more could you want? A remote control...

This was a rather cold and clinical sounding class A/B amp to my ears. And I sold them.

I would pair it with something very lush and warm. CJ Premier 3 or later, Eletrocompaniet, Meitner, Audible Illusions, MFA Magus (very good for the money). Pre '90's preamps would compliment the amp well. No need to spend a fortune.

The SP11 would be a little too towards the neutral for my taste.

How about the Threshold FET 10HL? I see them come up on Audiogon from time to time at around $800. Introduced in 1989, it is a contemporay match for the S/300. It originally retailed for over $3000 for the high level stage alone. Given proper warmup time, it is very neutral and quite dynamic with a very spacious soundstage.