This thread was an interesting read. The people who mentioned that there's not really any reason for you to officially quit are onto something. You've described a situation which is either coming to an end or at least slowing down significantly. Given that the gigs with this band will be few and far between, why not just work on your original stuff anyway? If the band does start up again and you feel the urge to quit, you can cross that bridge when you come to it.
That said, if you wanna quit regardless, I really don't think anyone here can tell you what's right because you know the band dynamic better than we do. I've been in about 15 bands, and each situation was unique. I quit some bands thinking that the in-person way was appropriate only to have some band members get pissed because they felt they wasted their time (and gas money) to get to the rehearsal space just to have me quit. I called the last guy I fired, and he was so petty/childish that it devolved into a ridiculous "conversation" -- really, just him making s*** up for nearly an hour. I had a feeling it'd go that way, and I wanted to just email him. Wish I had because I think it would've been better for both of us.
This opinion might not be popular, but I think too many people treat bands like marriages, and unless it's everyone's full-time gig which pays the bills, I don't see the point of making a big to-do about quitting. Sounds like the band you're talking about is a hobby cover band. Absolutely nothing wrong with that -- that's my current band situation -- and it's just not that serious. I've had a few personnel changes (people leaving and me letting people go), including someone who quit via text. Didn't bother me at all.
You just have to do what feels right to you.
