First things first - if you're going from a 55-01 to any of those basses, be prepared for an adjustment period. Unless you are already used to playing passive basses alongside your Lakland, it may throw you in the beginning. Passive basses sounded lifeless to me before I adjusted to the more organic sound of them, and started tweaking my amp a bit differently. The good news is once I adjusted, I fell quickly in love. Its a lot easier to find sounds that work perfectly in the mix with a passive bass than it is with an active. At least for me.
I've been playing short scales exclusively now for about 5 years. I've been through a lot of them and still own a bunch. Of the basses mentioned thus far in this thread I have a lot of experience with MIA and MIM Mustangs, the SBMM Stingray, and TMB30 mentioned.
If low end is what's most important to you, a Mustang is the clear winner. The MIM goes deeper than the MIA, but they're both great. The Stingray is my favorite (I sold all my Mustangs), but I like that funky, burpy sound more than the deeper darker ones. The TMB is a great inexpensive bass, but honestly - its a LOT thinner sounding than any of my other short scales. And I even swapped out the pickups for Geezers. That helped some, but it doesn't cut like the others.
2 suggestions before you start throwing money out the window

. Start researching active short scales since you're used to the Lakland. I have an active Warwick Corvette which is equal in quality and sound to any of my active long scale basses - but SS Warwicks are real difficult to come by. There are other actives out there though that just as versatile though.
And if you're going to go for passive, check out the Nordstrand Acinonyx (the cat bass). They may cost a bit more than what you're looking to spend, but they kick butt, and for reasons unkown seem to be a lot more fun to play than other basses. They're hard to put down once you pick one up. I absolutely love mine. As do most who buy them.
Hope that helps some.