While i can see a point in changing the combobox symbol, it think a + is the wrong idea. It usually adds content. The upanddown arrow symbol the clearlooks engine uses is better. And the increase/decrease buttons on the couter should stay as small as they are. Making them bigger would totally blow out certain UI, like in the Gimp.

I like the counter button at the right side. I think the blow out effect with the Gimp UI is a bit exaggerated. After opening Gimp, I noticed that the paintbrush scale and opacity settings reserved more space for the text than was necessary. You could easily separate the up/down buttons side by side, just don't draw them square, but rectangular with a 2:1 height:width aspect ratio. If this change still doesn't catch your fancy, then at least highlight the up/down buttons so I know which one will receive the click, because they are annoyingly small.