I recently flipped my 7-degree stem (from upward tilt) and dropped the handlebars down by a single spacer. [If it matters the headtube angle on frame is 71.5 degrees]

It seems as if my average speed relative to wattage has increased. But my neck and upper back are sore and stiff.

What should I try first?

a) flipping the stem back to +7 degrees

or

b) raising my bars by one spacer?

I just want your vote as to what to do first.