I think your barrels should be salvageable. Since you fished out the sulfur stick and are aware that you might have some residual sulfurous acid hanging about, you’ve already won half the battle; you know you need to do something to neutralize any acid in there. I would imagine you could get rid of most remaining sulfurous acid in your barrels by doing the following:
1. Put on protective goggles (as we all should when dealing with sanitizing solutions or chemicals).
2. Rinse a barrel well with water (warm or cold is OK, hot not needed) and drain the water out the bung hole. If your barrel still has pieces of the sulfur sticks or any other kind of solid residue inside the barrel you can see, you need to get it out. One of my favorite tricks is to feed a length of stainless steel chain (about 2 feet/60 cm long) into the barrel, add about 2 gallons (~8 L) of warm water, bung the barrel up and rock back and forth vigorously to help scour the sides. If you have heavy leesy solids that are dried on (though you should rinse these out prior to storage) sometimes the only thing you can do is fill the whole barrel up with water to soften these solids. If you have some kind of a very long-handled scrub brush that can reach interior spots, that can be helpful too. When the inside of the barrel is completely free of solids, proceed to the next step.
3. Introduce a basic sanitizing solution into each barrel to neutralize the sulfurous acid, using about three tablespoons of PeroxyClean, TriSodium Phosphate (TSP) or soda ash to 3 gallons (11 L) of water. Allow solution to contact all parts of the barrel by rocking back and forth, turning the barrel on its ends, etc. Do this for about five to ten minutes and drain.
4. Rinse well with water again and let drain for about three to five minutes.
5. Introduce a slightly acidic solution made with 3 tablespoons of food-grade citric acid to about 3 gallons (11 L) of water. This is to help neutralize any residue from your basic solution.
6. Citric acid is a food-grade and “friendly” acid that, with the above quick contact time, will do its job of neutralizing the basic chemical you added without soaking into the wood of the barrel with any level of significance, especially as you’re following with another water rinse right away in the next step below.
7. Rinse well with water and drain again. You are ready to fill!