Sub is huge on the club rig but vanishes on headphones

Played my tune out at the weekend and the sub was enormous on the club rig — but on my headphones it basically isn't there. Sub is mono, gain-matched, a sine layered under the Reese. Am I overcooking the low end for the room, or is this just a monitoring thing?

Bit of both. Headphones under-represent sub, so trust a spectrum analyser and a reference track rather than your ears down there. But if it only works on a big rig you've likely got too much sub-40 Hz — high-pass around 30 Hz and add a little harmonic saturation so the sub throws some upper harmonics and reads on small speakers too.