I would direct you to
K&K Audio. You can get a good Luhndahl-based box from Kevin. His prices are more than fair for these. There are two selectable gain settings on most of the implementations, so you can accommodate the output of most MC cartridges. There are also two sockets into which you can insert impedance matching resistors. You don't really need to solder them in, so you can experiment with what they sound like easily. Kevin has data sheets for all of the variants that he makes. These sheets include a table of resistor values to use to achieve the desired load. He would probably even throw in a set of resistor pairs that correspond to your needs based on the cartridge and the input impedance of the MM preamp. Further, there are several different options for grounding - one of which will be suitable if hum is an issue.
I have tried several different active head amps in the past and never liked any as much as a SUT with proper loading. You get noiseless gain with SUTs.
Finally, you can read all about this stuff and look up the recommended load for cartridges in the database at
Vinyl Engine