What tube pre amp to use

Audio Research has been reputed to work well with Threshold, but I haven't tried it myself.

With Silver monos. Pre was retired with MIT wire by dealer and was more extended on both ends and detailed but still tube sound. I want remote so am selling it off. Audible Illusions are also great for the price. Can never go wrong with AR stuff but I liked my Quickie once it was retired over the AR. Dealer had both lines and owned both for his own systems so I wasn't pushed one way or the other. I used it with krell amp at first and it made the Krell listenable as did the AR lol. Good luck

If you don't need remote control, there are two Primaluna Prologue 3 preamps now for sale on Audiogon. Does not have XLR out but does have dual RCA outputs. I own one and really enjoy it -- brings 3-D staging, warmth, inner detail, and overall sense of "aliveness" to the music. If you do need remote, check out Primaluna's more expensive models. The Primaluna preamps work best with amps with high input impedance, and if Internet info is correct (which reports your S500 with 75K input impedance), then you're in good shape.

Happy with my Modwright SWL 9.0 as a pre-amp...these are in your price range and if you are able to get one with the tube rectified power supply even that much better...Mine feeds my Tyler Acoustic Woodmere II's...

This is what I will offer. I have a amp that is comparable to yours. I have purchased a Consonance T1 preamp new for hundreds less than your budget. This pre is point to point wiring, audio grade caps, tube rectified with minimalist design. Available with remote volume only. No phono. Ok now the sound. In stock form good qualities one would expect from a well designed tube pre. To take this to very good I changed the rectifier with a Create audio ( got one for $50 online although they list $100) great rectifier. Also a mix of NOS tubes tesla and seimens now has unbelievable sound quality and clarity, wide open stage with pinpoint imaging, extended dynamics with good speed and solid deep bass. I took this preamp to demo it with the amp that I wanted to make sure of compatibility and synergy. Much to my and everyone in the room including the dealers surprise the T1 out performed the amplifiers matching pre which is very popular well touted and cost over $6000 . I was getting praise for the performance of this little unknown pre. This pre was better in every area than the 6k ss pre. I do not want to name the matching pre so not to ruffle feathers. If you are interested in what I compared against or where to purchase T1 pre email me. Best