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I'm in Michigan too and the combination of a decent fall and cold December seems to have thrown the bushes off kilter. Several of my bushes also have old leaves clinging to them. Since we had decent snow cover, which is great insulation for plants, I believe your spireas will be fine. I see winter die-back on my roses canes. You may have tips that don't green-up and these should be trimmed back in spring. Remember not to fertilize or prune for at least 6 weeks before first frost to encourage plants to prepare for winter. Somehow, that preparation message didn't get through last fall.

Spirea is a lovely bush. Pruning is best done in late winter/early spring before leaves appear. This also makes it much easier to see what you are doing! Old wood on flowering shrubs isn't as productive as young wood. The goal is to keep your shrub young and flowering by removing 1/3 to 1/4 of the oldest wood to the ground annually. Also, remove thin stems to allow better air circulation and light penetration. Remove suckers that are pushing out farther than you wish. Here is a one page reference for shrub care: http://web.extension.illinois.edu/cfiv/homeowners/030524.html

Magic Carpet Spirea.

https://www.gardeningknowhow.com/ornamental/shrubs/spirea/growing-spirea-shrubs.htm

It looks like the spirea have one of several leaf spot diseases known to affect spirea. You may be able to control the disease by spraying with Daconil. Since these fungi can over-winter in dropped leaves, it is important to clean up all fallen debris at the end of the season. Over-head watering is a risk factor for leaf spot diseases; a low-pressure soaker hose or drip irrigation is recommended. I would not immediately replant the same genus where there has been a disease problem. If this is done, keep a close eye for early symptoms of leaf spot and treat with an anti-fungal spray immediately. Repeat as necessary until any spores that spent the winter in the soil are gone. Of course, correct the over-head watering if this is the current practice.

Do you water these often? They will like a good dry period between watering, so standing in any water can leave them with a fungal infection.

The best thing to do will be to take a soil sample to your local extension service. They will be able to tell you what, if anything, if wrong with the soil and how to correct it.

This link will help you to find the closest one to you: https://www.gardeningknowhow.com/extension-search