I too have IHSS or HOCM. Ask your doctor about trying Norpase instead of Verapamil. I did not like the side effects with Verapamil. After a bout of Atrial Fibrillation in which Norpase was used to convert, they wanted me to take Cumadin to prevent clotting and stroke if I had A-fib again. I absolutely did not want the blood thinner and the Doc agreed to keep me on a maintenance dose of Norpase to prevent the A-fib. It has been two years and no more A-Fib. It is also commonly used to treat the HOCM itself with good results in some patients. It has done wonders for me as I no longer need the Verapamil and my gradient has decreased. I take Toporol XL and Norpase.
As you already known HOCM is a progressive disease process which ultimately may lead to surgery in those who are symptomatic despite being on therapy or those can't tolerate the meds. You seem to fall into both of these cathetegories. There is another medication called dysopyramide which is slightly better than the calcium channel blocker you are on, but its side-effect profile isn't any better. I think that at this point you should seek aggressive care including consideration for open heart surgery/myectomy. Here at the cleveland clinic we have very experienced specialist who deal with patients like you on a daily basis. Dr. Harry Lever is the HOCM specialist on the cardiology side. Dr. Nick Smedira is our surgeon who has done a ton on myectomies and other procedures/surgeries associated with HOCM. Mayo clinic also has a bunch of very experienced physicians.