I would say a Dingwall has the punchiest, tightest B string in the business. The runners-up would be Spector and Lakland IME.
Since you have a Lakland, which should have a VERY good low B, I would say to look at your choice of strings. You are already using a 135, you don't want to get too much thicker than that. Maybe you should look at the type of string. Hex-core strings typically have more tension than round-core ones do. DR Lo-Riders have a hex core and fantastic tone, plus they feel very tight. I have never had to use a gauge heavier than .130 on my 5ers, even the 34" scale models I've had.
But, if that doesn't satisfy you in the end, I would start saving for a Dingwall. They have a more affordable model (called the Combustion) scheduled to start arriving on stores in early '09. They really are fantastic basses!