With the fresh SF primaries I had 120 or 130 mA at the tailcap of my E2L Cree. ZERO, zippo, nada with a single primary.
A single 17500 fits the body width with ease (yeah I know it's too short!), @ OCV ~4.05 vDC, fed 200 mA into the KX2 while producing what appeared to be a normal amount of light.
Regarding available cell space, the KX2 must be fully screwed on to work, and there is no spring at the bezel end. The "shelf" between the negative end of the cell and tailcap will be a hard constraint on the cell length...would a 17670 be a "crush" fit between the bezel and "shelf" and screw up the protection circuit?!
ETA:the OP metioned 17670 not fitting, but was not specific why. Too long?
Paladin