What kind of meter are you using for measuring the resistance? The speaker secondary outputs will measure as being in the "fraction of an ohm" range, and the "common" side (black posts) is grounded, so all 4 of the speaker binding posts will appear to be "continuous" (shorted) unless your meter / DVM has a low-ohms range, in which case they should show about an ohm or slightly less from red to black. If indeed they were shorted, the sound in the headphones would be non-existant or very low and muffled at best.
Since you do have good sound in the headphones, it would appear that everything is working properly at least up to the headphone jack. That would leave as being "suspect", the wiring between the headphone jack, and the speaker binding posts. Double check the wiring between the HP jack and the binding posts for correct connections, and for solder joints.
Hopefully that should fix it!
(I've built two of these amps, one with the HP jack, C4AS boards, iron and cap upgrade and the other one as bare-bones "basic" without the HP jack, OPT 8 ohms secondary connected direct to the binding posts, for speaker-only usage. (Headphones could be connected via a banana plug to 1/4 inch stereo jack adapter, if required)
/ed B in NC
