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.
