You can get a soil test. Google Logan Labs in ohio. How did the pro determine this?

Get a good soil test or else don't worry about it. Most of the grass is growing. I can almost guarantee you the C marks have nothing to do with pH.

Where do you live?
What kind of grass do you have?
How often do you water and for how long?
When were the last two times you fertilized and what did you use?
Have you reseeded recently?

I live in Ottawa, Ontario, Canada
Grass is Kentucky bluegrass
Water the lawn 25 minutes a day using automatic sprinklers
Re-seeded twice in early spring with no apparent sign of success
Lawncare company has fertiilized in the Spring, but I don't know the details. They didn't do any soil tests to come up with the diagnosis of high ph. But I use the same water source to fill the hot tub and chemical tests for the hot tub water have been coming back with high ph.

Water the lawn 25 minutes a day using automatic sprinklers

There is more than likely your problem. You are drowning your grass and likely have fungal and mold problems. In your area once a week is more then likely way too much water.

Agree with TW. Normal watering by Mother Nature is once a week at the most. You should check your sprinklers with a tuna or cat food can to see how much water is applied in 25 minutes. The goal to start with is 1 inch per week all at one time. The very worst thing anyone can do is water every day. This is sometimes a hard lesson for people to learn, but it is the single most important thing you can fix. Watering should be deep and infrequent. In the cool months you should water more like 1 inch per month all at one time. If it was your gardener who fixed you up with that watering schedule, you need to let him go and find someone who has a better handle on professional turf management. I am an infrequent contributor to a professional turf care forum, and their members were the originators of the deep and infrequent watering rules - at least as far as I can determine over the past 10 years of doing this. I fought them long and hard to disprove the deep and infrequent theory, but they won me over. I wholeheartedly endorse it now. It really works and solves a lot of lawn issues.

Spring is a terrible time to seed; but more importantly, KBG does not need reseeding. What prompted you to reseed at all? If it was the lawn care company, you really need to let them go. KBG is famous for never needing to be reseeded. It is a sod forming grass that will spread to fill in every thin spot before you ever see it become bare.

Your grass patches that look like a C are really grass patches that look like an O except part of the fungal ring is not complete. You have a disease which results in a circular ring called a fairy ring. This could easily be caused by far too much water in the soil. Getting rid of it might be as simple as correcting your irrigation schedule. A disease is about all that could take out KBG and cause a bare spot. Before you go to the trouble of finding a fungicide, please simply correct the watering and see what happens. Fungicides may or may not work on specific diseases, but they always take a toll on the beneficial fungi which your soil requires for good health.

Your lawn care company could know that you have a high pH simply because all the soil in your area has a high pH. My area (about 2,500 square miles in all directions) has a pH of 8.0. It doesn't take a genius (or even another soil test) to figure that out. But if you are successful growing KBG elsewhere, it is unlikely the C spots are caused by high pH. Still, if you would like a really good soil test to pinpoint any and all your soil issues, send a sample to Logan Labs in Ohio.

I agree with the recomendations above but to answer one of your origional questions,prilled sulphur is slow acting but a cost effective way to temporarily bring down the soil pH

Old thread alert, but nobody caught the fact that lawn companies use anti-germination sprays that were part of OPs failure to reseed.