Spot on about the XTherm-I’ve got the Quest 6.0 version (similar R at 6.5) and it transformed my quilt setup from “bearable” to “toasty” on GSMNP shoulder season trips where ground temps hit 25°F while air was 40°F. UberLite’s thin foam matrix conducts cold laterally if you’re a side sleeper; quilts amplify that since there’s no bag wraparound.
Quick pad science for quilts: Aim for R >5 in variable conditions like Smokies (EN 13537 comfort equiv). Test your setup dry first-sleep system rating = quilt limit - 20% + pad R factor (rule of thumb). My Revelation 20 + XTherm floors at 25°F real-world; drop to UberLite and it’s 35°F misery.
VB caveat: Polycryo or Stormproof works but pair with merino base only-no cotton. Sweat wicks out pre-bed, then seal. For OP’s quilt, try Sea to Summit’s own Reactor liner first (synthetic, $100)-bridges to full VB without commitment. What’s the exact Spark model and pad? Numbers sharpen advice! ❄️