We planted them in poor soil, which is why I want to 'feed' them with something.

Don't fertilize now. Only fertilize in the fall before they come up.

Actually, from the link below, it says,
Guidelines for Growing Daffodils
"Top-dress again with 5-10-10 when the leaf-tips emerge. As they flower,
top-dress with 0-10-10 or 0-0-50. High-nitrogen fertilizer should be
avoided."

Another good read is Non-Blooming Daffodils.

#1 says, "

1. Bulbs have not been ‘fed’ in a couple of years (a broadcast of
   5-10-10 granules at planting, when leaves emerge, and again at bloom is a
   reasonable feeding schedule.

hth.

Sue

edited to add. The use of ' all purpose' MG may or may not be, all right to use. It would depend on the N-P-K percentage of the MG. You do not want to use something with a lot of Nitrogen (bigger first number).

I've noticed recently that some, if not all, of the new Miracle Grow products no longer show the N-P-K prominently on the front of the packaging. I would guess that it would likely be found somewhere in the fine print on the back.

I have grown daffodils for 40 years. I have many friends who grow daffodils to show. We have never fertilized once they are coming up. However, you must follow whatever recommendations you feel are appropriate for you. One thing you also need to know is that if daffodils are not blooming or blooming sparsely, they need to be separated. Dig the clump at the end of the blooming season and look at it to see if the bulbs are matted together. Separate to make smaller clumps and replant.