Could a heavyweight KO a welterweight with a jab?srs

Being that a heavyweight jab is like a cruiserweight right cross, the answer is yes. A jab from most any heavyweight could knock out a welter. It wouldn't, most of the time, but definitely could.

maybe even a middleweight

Absolutely. Without a doubt. Anybody who says no is a certified idiot, IMO. There's weight classes for a reason.

An Audley Harrison jab, no, but a good solid jab from most heavyweights would at least send a welter onto queer street.

Well i weigh 235 and I put a LHW on his ass with a straight jab down the pibe, and he had helmet on, i have also broken 2 noses, so yes My jab is heavy and fast, a right hook perfectly palced on the head from a fully fledge HW would end the fight

Sonny Liston's jab would separate Mayweather's head from this galaxy.

I would assume so. I mean a WW KO'd another WW with a jab. Kell Brook did it to an Argentinian fighter who's name slips my mind ATM.

Larry Holmes, wlad , or foreman I think could ko a ww with a jab.

I don't know about a KO but definitely a knockdown.