IMHO, this has some chance of hurting OP, depending on how similar the "new direction" is, how litigious they are and how successful OP is in what he does next.
I'd stick to "personal reasons" or none at all.
0. Get your CV out there
So you can start some leads and if they ask for some weeks notice, you'll have something going (at least some leads) when you're no longer there
Find a new job, then give notice.
I've worked in this field since 1995 and am an employer at a fairly sizable team now and I'll tell you this is pretty close to industry standard. Competent employers do not have an expectation that they're going to get a heads-up before you start interviewing.
1987. My sister didn't know there was a root user on a Unix system. I found out she was interviewing after seeing her resume in her home directory.
We hired him. He was absolutely fantastic.
Or maybe because she had a problem I was in her directory and saw a suspicious file.
More than likely I was doing recon - I'm curious, so anything is possible.
Also I'm not sure how the existence of a root user plays a part because on the vast majority of unix systems the default home directory permissions are permissive read (this was true in 1987 and is still true now) and if your sister didn't know about root users it seems fairly unlikely that she knew about chmod.
If you are uncertain, and your current employment situation is feeding that uncertainty, it might be better to get out. I say this as someone who didn't really realize the way I was being effected by my work situation until I heard from a few people who left ahead of me.
PS if they throw more $ at you, don't bite. Chances are you will quit again within 6 months. Good luck!
That would change your relationship somewhat, going forward, so don't do this unless you actually would be content to stay at the higher salary longer term.
Often, many of the most difficult decisions in life are not what you should do - because deep down, you already know - but how do it. This is true for your decision right now.
A few thoughts for you on that:
1. Irrevocably commit to doing it. Book a meeting with the founders to discuss "something important". Or tell your significant other that you've decided to exit by a certain date. Just do something that commits you to this course of action.
2. Get creative about why you're leaving. Instead of leaving because the company is a sinking ship, think of a diplomatic, but still truthful, reason for exiting. For example, focus on what you are going to do next, and supply that as the reason: "I'm much more interested in working on this kind of technology/product/etc. and I want to go pursue my dream."
3. If you can't bring yourself to tell them your decision face-to-face, put it in writing. Email them to tell them what you've decided, and close it with, "Can we talk?" Then hash it out with them. This is a great way to be done with something (see #1) because once the email is sent, it's sent.
4. Always remember that it is never as bad as you think it will be. It's just business, they will understand your reasons if they are reasonable businesspeople too. If they're decent individuals you'll stay on good terms with them. They'll miss you, they'll be "disappointed", but they'll get over it. And don't be surprised if they've already expected you to do this months ago. It is usually obvious to employers when their employees are unhappy or frustrated.
You already know what to do. Now go get it done! Best of luck to you.