Hey Kristina, I haven't watched that many k-dramas but I happen to like ahjussi-romance :) I assume you are serious about "obvious age difference" and not just "he is a bit older"...
What I can tell you is this:
* Beethoven Virus: Kim Myung Min is one fine ahjussi indeed. <3 In this drama, his character is acting sooo terrible especially in the beginning but there's a very human side too and if you're open to trying to understand him, you will fall in love very soon. It is a wonderful and underrated drama. <3<3<3
* The King of Dramas: ehh, okay, KMM again in a sort of similar role actually. I think maybe some people like this one better. But it's good ahjussi romance. Even if it takes some time before they get there, you will enjoy the ride for sure.
* Valid Love: a high school student and a teacher fall in love, don't start a relationship but meet again 12 years later. So she's grown up, but he's still an ahjussi. ;) I think they're a great couple and I always rooted for them, but the drama is a little bit draggy because of side stories with supporting characters. (Disclaimer: I watch mosty j-dramas so I'm used to shorter dramas... some people may not find it draggy.)
Also, some j-dramas (even though you posted in the k-drama forum) that you may find qualify:
* Iki mo dekinai natsu: mostly subtext but I really enjoyed it!
* Kou Kou Kyoushi 2003: teacher/student, one of my favourites
* Mitsu no aji (oh, but there's angst!)
* My Rainy Days (movie)
I wish I had more to tell you! ;) This is all I know. Do you have any personal favourites that I haven't watched yet?
I see that you like Painter of the Wind... it's on my ptw-list. Do you count this one as "ahjussi-romance"? Personally, I generally prefer male leads around 40... or at least in their 30's.(With exceptions, though... but if we're talking preferences. Like, if I was watching a drama about a 20-year-old and a 27-year-old, I can see why people would call it "age gap romance" but I wouldn't be able to think of a guy that age as an ahjussi. I mean, it could still be a good drama of course...)
Also, I have to disagree about Nine Times Time travel. The drama surely is AWESOME and I love almost everything about it including the romance, but I never thought of the age gap as very important even though it is true that the girl is younger. (Disclaimer: maybe it's because I'm old enough not to think of Lee Jin Wook as an "ahjussi". If you're around 20 maybe you do. He's really hot anyway.) It is a bit angsty because of another reason than the age. I think you should watch it though!!