Just to be the "devil's advocate" here, why try to change what you're doing?
You look great... Probably 99% of the male population would kill to look like you do right now without any added mass.
Honestly, that will just accumulate anyway as your muscle matures. If you aren't satisfied and think you need to pack on mass, you are opening yourself to shoulder and knee problems from being in that mindset that tells you that you have to keep pushing those numbers up.
Stay active, lift like you have been doing, but maybe even back off a little at this point. Muscle is pretty easy to maintain... hard to gain. You've gained it, so maintaining will not take gut-wrenching exercises every day like you've probably done so far.
I'm just throwing out other alternatives. We are all so constantly focused on what we need to improve that it seems we rarely get to a point where we say, "you know, I look pretty damn good. I'm going to try to keep looking like this and be satisfied with my physique."
Maybe that's just the age speaking.
Good job so far though. If you throw out any of my pondering/advice, at least stop and take a moment to be proud of yourself for what you've accomplished so far.