As you stated yourself, you can't rely on the LCD to determine exposure. The LCD is going to look fairly bright when shooting in an inside studio. The histograms for both of these photos are similar. The lighter colored guitar may be causing the camera to underexpose some. Use the histogram on your camera to determine where the exposures are at. Neither shot is unusable, but could stand a little more exposure.
Just to add, I can see the detail in the head of the guitar in the bottom image. Is your monitor calibrated, or has something changed with it?