Unfortunately, by my own preference, I am anti-spam, thus, I don't spam, and not in favor of spam method.
I don't dispute how effective spam is, but there's always a positive and negative, always take both into consideration.
As what
@Reize mentioned, I prefer to let people watch or sub or share on their own free will. I appreciate quality over quantity.
There's no point to grow without substance, like a hollow tree without the inner support. If your subs base is mostly just subbed because you spammed or begged them, not really because they're even interested, you don't really get the audience retention, the proper following, etc. That's what I feel anyway.
I am not saying to not grow, or not grow fast, it entirely depends on what you are looking for.
Good luck in your venture, and hope your objectives are achieved!