I have a 406 with Trick Flow 23 degree heads, Performer RPM intake, XE274 camshaft, and a 750 Holley that I just put on last weekend. I currently have the white spring (lightest spring you can get) on the secondary diaphram. I have great low end response. Even with the 2.73 rear end and the four speed, mine doesn't bog at any RPM. I can easily lug mine around at 1200 RPM in fourth gear. I can stomp the gas at 1300-1500 RPM in first gear and melt the right rear tire (posi needed bad).
What are your cam specs? What kind of intake do you have? What gears are you running in the rear? A big cam and a single plane intake with high gears would perform as you describe. What did you have on it before and how did it perform?
When you say "bog", what do you mean? Does the car accelerate good in third then bog when the secondaries open? If that were the case, you would need a heavier secondary spring. Or, does it just not want to gain RPM? If that were the case, you might want to try a lighter secondary spring and/or look at the way your carburetor is tuned more closely. Another possibility is, like Dennis suggested, your engine, with that particular carburetor might not be in its power band below 2500 RPM. The higher gearing of third gear could just be magnigying the problem.