What tube pre amp to use

There is a nice Quicksilver listed here for $575,I recommend this since I own one and think it is real nice. As far as output for sub all you have to do on any preamp is to add a y adapter to the outputs.

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...