There's your answer right there. Sorry, but there's no magical workout plan that anyone can give you that's going to offset the unwillingness to eat enough food to grow. Either decide to be willing to eat enough to achieve mass gain, or choose another goal.
I agree with Nate Dogg that getting a copy of Berardi's "Scrawny to Brawny" would be a great investment. But, I'm going to tell you that in a nutshell his advice is the same as the advice that I just gave you above.
You need to eat more, and be willing to eat as much as is needed in order to gain muscle. If you're not willing to do that, then you can go ahead and keep training, change up any parameters that you like, try every routine out there, but you will never get the muscle gain results that you're after.
Why would you only be willing to add 500-1000 cals/day? Also, if that has "never seemed to lead to serious gains" for you, what makes you think that it's going to now? Guess what, if 500-1000 cals/day haven't produced gains in the past, then you're going to need to eat more than that in the present/future in order to produce gains.
Sorry if this reply comes off as harsh, but you are in serious need of a wake up call.
There is an old saying that goes "Winners are simply willing to do what losers are not." So, you need to make a choice, are you a winner? Are you willing to do what it takes to reach your goals? Or, and I hate to say this because I'm sure you're a nice guy and I'm not trying to attack your character, are you a loser? Are you doomed to failure and mediocrity for the rest of your mass building career due to an unwillingness to do what must be done?
If you decide that you're really not willing to do what it takes, that's ok. No one is going to ridicule you for deciding to follow a different path (maybe you decide that you'd rather get really "ripped" or participate in some sport and dedicate your time and effort to getting better at it).
But, your tone in your OP suggested that you are unhappy with your current level of muscle mass and that you feel almost ashamed that you train as hard as you do, and for as long as you have been, and yet "don't even look like you work out". That sounds to me like mass gain is something that you really do want.
If that's the case then seriously, take my advice, and take Nate Dogg's advice and pick up a copy of Berardi's book. Like I said, the book is going to tell you basically the same thing that I did above. But, it's also going to provide you with a plan to achieve your goal.
Good luck and good training,