Sounds as if you've checked already but it could be that the factory
ground is faulty. IIRC, the factory ground for the headlamps merges both
high and low beam ground wires into a single spade terminal at the
grounding post above the fuse box (via a double-male to single female
connector or some such). My memor is hazy, but I believe there are only
one or two setups like this at the ground connection, so it shouldn't be
too hard to locate and test.

If a headlight upgrade was performed, that may mean that a relay was
installed. Some upgrades use a single relay for both high and low beam. If
this was done in your case, it's possible that the relay is faulty or lost
its ground, hence both high and low beams now inoperable.

Look for a relay near the fuse box with blue, yellow, red and either brown
or black wire connected to it. The problem may be the power circuit on the
relay (input or output, usually terminals 30 or 87, or the ground for the
switching circuit on the relay (usually terminal 85). Good luck!

- Kevin