>I've found that when I increase the brightness on an underexposed photo, it helps a lot to increase the contrast as well. That restores some of the apparent sharpness to the image. I don't have any rule of thumb on how much, I just play with the controls until it looks best.
It's hard to tell exactly what you are doing except for the ones where the flash is used. It creates a shadow on the left and a highlight/glare on the inside. I'd do away with that.
Basically your light is too even. Most of the time we have the opposite problem. The left and right are the same brightness. Try either moving one light further away or shading part of the light to make it dimmer. You may even be able to aim the light a little less directly at the piece which will also dim the lamp a little.
This will create a subtle shadow on one side and make the other side appear brighter. This makes the piece look more 3 dimensional and also helps it look sharper. For interest sake turn off one lamp and look how much sharper the piece appears.
As Photographers we constantly play the game of creating shadow to enhance sharpness and texture without making the shadow ugly. It can be challenging.