When I had my gibralter I alway used a baro(but I only ever burned coal), when I got my new DS Machine hand fed the factory told me to use a MPD. This was new to me so I asked some questions. Now what I am gonna say is what the maker of my stove reccomends, each manufacture is differant so check your owners manual.
DS Machine said they use the MPD and it works extremely well. he claimed you don't need that much air flow out with coal and once the fire was going, and after refueling and the gasses burned off to close it completely. Even when it is closed it is not really closed with the holes in the plate and the gap around the side. He said unless people have a manometer they normally don't set up a baro well enough and lose heat up the chimney. That is another point if you don't know your dratf you really need a manometer, to set up a baro properly. I asked him if I could run both and he said that is the best(now bear in mind this is for there stove again). So that is what I did. I have the stove, mpd, baro, then up the chimney. I must say it works extremely well I can feel more heat when the MPD is closed. I got my stove at the end of the season, so I was not really burning it to hot, more of a low fire, and after reading all the posts in this section I thought I am going to do a little experiment. So now I have a test port before the baro and one before the MPD. Out door temps were in the high 50's I checked my draft before the baro (with the mpd open and closed made a negligable differance) and it was -.06, Now I checked it before the the MPD and it was still -.06. This was interersting. This went with what they said at DS Machine. Now Like I said that was on a lower fire, this comming winter when it is really cranking I will retest and repost my results, I expect with the larger fire and more combustion gases comming off there may be a slight drop, but I doubt it will be much. The baro keeps your draft constant which is really nice, and the MPD keeps heat in the fire box. So I guess now I am for having both of them
if your manufacture alows. If you want to burn wood then wire the baro shut until you are ready for coal. The thing with the baro is if you have a chimney fire it will open more and add more fuel to the fire. I would put both in and use both(if you can)or only use the one that your manufacture recommeds for the fuel your burning at the time. I hope some of this helps these are just my experinces, and remember check with your manufacture for proper operation.
Have a great day, Eric