After seeing numerous posts about how the current SPL meters from Radio Shack don't measure flat, I fell upon a post that sent me to this web site:RS SPL Meter Mods
With just some soldering experience, you can modify your SPL meter to get a flat response. I happened to have 2 of these meters, one an older model number, and the current version(both analog). After the mod, when checking my sub level with Avia, the unmodded model was at zero, while the modded meter now showed the same signal at 2db higher, just as anticipated.
Worth a try. Just try to get 10V versions of the capacitors. I removed the old ones and replaced them, where as the instructions give you the option to just solder them onto the back of the circuit board in parallel with the existing caps.