I purchased a pair of blue heron's on Agon with the tweeter problem disclosed. They sounded exactly as you describe with a lack of high frequency dynamics or general loss of volume. I previously owned a pair of heron i's and was able to easily identify this difference through comparison. My solution was to replace the tweeters with Scan Speak ring radiators. This process is involved but excellent results can be attained. You'll see that scan speak tweeters were used in the BH2's. First you will need to chose the Scan Speak model that has a similar diameter to the HD3P you will likely need to expand the tweeter opening in the box a few millimeters. Next you will need to get into the crossover which can be easily accessed through removing the bottom plate. You will notice that it is a 1st order crossover and thus relatively simple. Next you will need to calculate the resistors needed to properly adjust the sensitivity in the new tweeter. I started with the Sensitivity of the HD3P and its current resistor/s to find the existing final sensitivity. Then built an resistor network around the scan speak to result in the same final sensitivity. Additionally, the HP3P makes use of a transformer which will need to be removed. Another option is to use a L-pad which is basically a variable resistor that you can dial in to your liking(Would require more box surgery and would likely be sonically inferior). Hope this helps it worked our very well in my system.
