Well I haven't been at 6k subscribers forever. For the first 4 or 5 months, I got about 80 subs total. Then I got on the daily comic snack, jumped to 300 subs, and have been on the front page frequently for about 7 months now. Over that time, being on the front page has never given me more than 1 or 2 subs a day. Getting from 300 to 6k has always been from large boosts (usually groups of 100-300s) from Daily Comic Snacks, being featured, or being linked from another site.
Being on the Main Page is great for your image, and I'm so grateful every time I see The Other End up there. But it doesn't necessarily make a huge impact on the numbers.
Also, the idea that the app is such a different entity from the site is a very new phenomenon. There's still a lot of reader overlap.
Of course, all this is just my personal experience. It's gonna be different for everybody. But like everybody, I'm still trying to grow my comic as quickly and consistently as I can.