I recently spent a bunch of time designing a silencer system for my business partner's RV generator. The end result was that I was able to knock down the 3600rpm Onan generator noise emission from 93dB to 83dB (at 10 feet) with just a little bit of effort. During this, I got some tips from my father who spent his career as an acoustical engineer. Key things are:
1. mass is your friend. If you have the ability to surround your dyno cell and all piping until the actual exit with as dense a material as possible, that is best. Burying system underground or behind layers of 5/8 plywood and 1/2" sheetrock also works. There is flexible sound absorption matting available at a reasonable cost too.
2. you are dealing with variable frequencies, but you can still attack them with a combination of Helmholtz resonator plus a multi band pass filter (muffler)
3. There are some good Helmholtz calculators out there. I made an Excel spreadsheet for mine and built the resonator as a small pipe leading to a larger plenum, with the small pipe mounted 90 degrees to the exhaust pipe. The key formula that you want to work with is w = c(sqrt(S/(Leff*V))) where w is the target frequency, c is the speed of sound at temperature (I used 4200 inches per second), S is the neck cross section area, Leff is the effective neck length and V is the chamber volume.
4. There are a number of muffler designs out there and a google search will quickly bring up different options of how to design a muffler that will target multiple frequency bands, if you want to build your own.