My dog has SAS. Is it safe to give heart supplements with Sotalol?

The biggest issue with herbal medications (which is what this supplement is) is a lack of controlled studies to prove that they work. Additionally, just like a man made medication, an herbal medication can also have side effects. Having said all of that, the specific product you refer to does contain Hawthorn which does have effects similar to sotolol. I don't know that its "better," or "safer" than sotolol because we just don't have the data to show that, whereas sotolol has been studied extensively regarding dosage, safety, efficacy, etc.
In short, its reasonable to try this herbal medication but I would strongly advise that you do it under the care of a veterinarian who has been trained in herbal medicine. Here is a link to find a a veterinarian who has studied herbal remedies: https://www.vbma.org/us-members.html
Keep in mind that SAS is a mechanical defect, and so no medication will "fix" it.. rather the goal of medical management is to minimize the problems that the heart defect causes.
http://massvetcardiology.com/Data/resources/cardiacdiseaserefe/subaorticstenosiss/understanding_sasgeneric.pdf

Best of luck with her!