Looking good, but at 155lb and 6' tall, I have to wonder how much muscle mass you have elsewhere. I'm getting the feeling that your legs, and possibly back, will need a lot more catchup work, as I wouldn't expect many lean guys your height and weight to have that much visible muscle. But I could be wrong. Either way, if your goal is - as you say - to gain more mass, then developing the whole body is definitely the way to go.