As far as firing is concerned (if you are definitely going to do it), I suggest either firing them and then waiting a couple weeks to try my suggestion below OR trying the suggestion below, waiting a probationary period of 3-4 months to see if they improve, then firing them. You definitely don't want the firings to be perceived as intimidation or retaliation.
Meet with the most mature employees
I agree with @ChrisG that you should probably sit down with your most reasonable employees - the ones who can put their emotion aside and who also feel confident enough to talk to you. Explain the situation calmly with neutral language that doesn't blame your employees or yourself. Something like, "I wanted to meet with you today to get your feedback. It's my impression that the team is somewhat dissatisfied with the work environment here. Can you give me some specific feedback on why that might be?" Then sit back, let them talk and don't get defensive. They could say things that seem ridiculous and inflammatory. That's ok, just write them down.
Get specific feedback on behaviors or actions they perceive as negative
Very often, people will talk in emotions when they are upset, but what you will want to do is drill down to specifics. For example, if they say "we feel like you are really intimidating to talk to", ask them (again, without being defensive) "can you give me specific examples of things I do that make you feel that way?". What you want is a list of the behaviors they feel threatened by. When you have pretty complete list compiled, thank them for their feedback and end the meeting cordially.
Do some soul searching
Now comes the hardest part to my mind: think long and carefully about the list of behaviors they see negatively - how would you feel if you switched to a field where you were not the expert you are and someone exhibited those behaviors towards you?
Make any necessary corrective actions
Hopefully this process will allow you to identify behaviors you want to change and you can set those goals either privately or with the assistance of your staff.
There is of course a small possibility that all ten of your employees just happen to be terrible people, but especially given the phrase "I have a history with staff wanting to leave usually after about 15 months" suggests the odds
are pretty low.
A suggestion based on my experience
I'm going to go out on a limb and give you some advice, based on my ill-informed impression of the situation. I probably don't have as much experience as you at managing and you should certainly take my advice and example with a grain of salt. But I hope it helps anyway.
When I first became a manager, I tried to micromanage the way my employees they did their work. This was terrible for morale obviously, and led to (a) me complaining that I always ended up having to do their work for them and (b) them feeling like I was belittling them and criticizing their work (which admittedly I was). It is 100% true that I had many years more experience than they did and the methods they chose weren't nearly as efficient or effective as the methods that I micromanaged upon them. But you know what I realized? It doesn't matter. At the end of the day, what matters is that deliverables are completed on time and the results are correct.
I decided instead to start managing the deliverables of their work. That is, I would specify what deliverable we needed in what timeframe and let them decide how to accomplish it on their own. I put the emphasis on correctness and timeliness, and as long as I defined the deliverables well, the employees would generally deliver. Oftentimes (especially at first) the methods they chose weren't the best, but as long as they delivered what I asked for, that was ok. Occasionally there would be times when they would miss on correctness or timeliness, but I would emphasize that they should come to me early and often if they felt they weren't going to deliver. This allowed me to facilitate by offloading some of the work (if it turned out to be more than expected) or offering suggestions on a better method (if the problem was the way they were going about it) or manage client expectations (if there was really nothing that could be done about it).
With the employees spending more time thinking about how to solve the problem instead of simply enduring my critical micromanagement, they started coming up with better and better ways of getting things done, which ended up solving the problem I originally had with them when I was micromanaging!