Reverse. Raising the low tide mark means that the spin up is triggered when there is more music left to play in it.
Under optimal conditions, the low tide would just cover the worst case time taken to spin up the drives and read in new data - this would allow all the cache memory to be refilled allowing the longest possible time between fills.
Raising the low-tide mark increases the amount of cache 'reserved' to cover this spin up and helps for non-optimal conditions. However, in optimal conditions this extra amount of cache never gets emptied, thus reducing the total amount of cache filled at each spin up, hence reducing the amount of time between and increasing the frequency of spin ups.