This is an interesting question which I've investigated at length. I have been very involved in purchasing and setting up drums for several churches and also for a coffee house with acoustic music. I also play with an acoustic folk/roots music trio. In all cases volume was a huge issue.
Here's what I've found: A full snare hit with rimshot can reach a volume of 120-140db, an average volume snare hit ranges between 100 - 120db, just for reference a gunshot is about 140 db and a jackhammer is between 95 - 125 db, average un-amplified piano volume is between 65-75 db, and an un-amplified vocalist from about 3 ft. can reach 65-75 db.
Your goal should be to reduce average drum volume to between 65 - 95 db with peaks for accents and crashes. (for example hi-hat or ride cymbal, which is sustained should be at 60-70db while snare hits or accent crashes may reach 85-95 db
Use of a clear shield, internal muffling, different heads, Will only effect volume minimally 2-10 db reduction. Hot rods, thunder rods, whips, and other specialty sticks can reduce volume by 10 - 20 db (this is of course dependent on technique ie; full force snare hit with Vic Firth Hot Rods can reach 110 db.)
In our experience only 2 things can reduce drum set volume to a level which will comfortably complement acoustic guitar volume with minimal amplification.
1- Create an entirely enclosed "box" complete with sound absorbing foam sections around the kit. We did this at a church and were able to reduce the volume of a full blown rim-shot to about 65 db and ride cymbal volume to 40 db. We then mic'd drums and had complete volume control for church services. (you will also need to deal with air flow and temperature issues)
2 - Drummer Control! We found that drummers with control can significantly reduce volume, changing heads, using smaller sticks, hot-rods, muffling, will all help a drummer but ultimately asking drummers to learn control can reduce volume by 20-35 db. Also using alternate drums for acoustic music, ie; hand drums, snares off, mallets, shakers, etc can greatly reduce drum volume.
in Conclusion, short of putting your drums in a box, drummer control is the best method for volume control. Anything else will not make a significant reduction in volume.
(Again there are methods of reducing volume significantly but they also change the sound of the drum set completely. Our goals was to keep the drums sounding like drums. An example would be to cover each drum with a towel, or completely fill the bass drum with foam or blankets.)
I think everyone's making the same point; it's the drummer!
That's what I've found.