This is lengthy, and feel free to refute me if you have proof to the counter, but I think my theory holds up...
To "make a crab happy" you need to play the musicblock it has heard the least recently. Not including the SB-1, there are 26 musicblocks (5 of each type plus the GnG). When you play a block, the crab will either say, "I just heard that one!" or give you between 50 and 650 currants, the number going up in increments of 25. It is theoretically possible that the crab gives out currants randomly, but I believe the currant amount is attached to how recently the block you are playing was played before. Like this:
1st block (played MOST recently) - "I just heard that!"
2nd block (played second most recently) - 50 currants
3rd - 75
4th - 100
5th - 125
6th - 150
7th - 175
8th - 200
9th - 225
10th - 250
11th - 275
12th - 300
13th - 325
14th - 350
15th - 375
16th - 400
17th - 425
18th - 450
19th - 475
20th - 500
21st - 525
22nd - 550
23rd - 575
24th - 600
25th - 625
26th - 650 currants
27th (only possible if the SB-1 is included) - 2000 currants plus the achievement
Support of my argument:
I tried to grind this achievement over the last several days and ended up playing about 150 musicblocks randomly for all 6 crabs (i.e. 25 random musicblocks to each crab). The randomization is important. The odds of each individual block may vary but so long as there is always 1 correct musicblock (the one played the least recently) and 25 incorrect options, I should have a 1 in 26 chance of playing the correct one (a roughly 3.8% chance). I did not get the achievement, but I did get all the other amounts of currants plus the "just head that" option roughly six times each (I received 425 currants only three times and 225 currants 8 times, but you get the idea).
Sure, it's possible that I was just really unlucky, but if you look at the top ten achievers, NONE of them (including myself) have the "make a crab happy" achievement. And these are people who grind hard, who sit in their backyard, killing themselves with no-no powder for hours just to become Dracula. Odds are good they've tried to grind this one out as well and have failed. And then of course there are the masses of glitchen who have been playing for months, and very few of them have the badge. That kind of widespread "unluckiness" just doesn't seem possible.
Common faulty counterarguments:
1. "I have the achievement, and I got it playing some random musicblock, like DG-2!"
There are a handful of glitchen out there with the SB-1. Thus, every time one of those glitches plays an SB-1 for a crab, 26 other glitchen get to use the other boxes to get the achievement before the crab needs to hear the SB-1 again. If you ask on global chat or in the forums, you'll usually find a few who do have the achievement, but not nearly the number that should be proportional to how many people are TRYING to get the achievement.
2. "Your statistics are wrong. The odds are much less than 1 in 26, because some musicblocks are played often and can never be the correct choice."
Some musicblocks ARE played most often and have a lower likelihood of being played most recently. An average glitch may play the musicblock she thinks is most rare, the higher numbered boxes because they sell for more, or the BB-5 or GnG because they actually are rare, meaning these probably have lower odds of being the best choice. You'll have a hard time getting the achievement with a GnG because glitchen with GnGs may just carry that one around as their one musicblock to play. BUT, so long as you select your musicblock at random, and because there MUST be at least one correct musicblock, the odds should stay firm at 1 in 26.
So... if there's anyone out there with an SB-1, let me know if you got the achievement with your SB-1. Or heck, I'll pay you 40k just to borrow the SB-1 for a minute to use on a crab and test my theory.