Based upon a lot of research, we have just completed our move to Nimbus Hosting as a step up from previous shared hosts. I am very happy to say that all the many recommendations we received (and some here: http://xenforo.com/community/threads/time-to-change-hosts.36640/) are all absolutely in line with our fantastic experience with the company.
Nimbus and the service it provides couldn't be more different to our previous hosts. To our previous hosts we were just a number, and service wise, very low priority. With Nimbus, even from the very first email, we have been made to feel such a welcomed and important customer. Since our moving to Nimbus, the service levels have quite simply exceeded our every expectation, even to the point of helping solve issues outside of our hosting, using the expertise on forum software. For the first time in a long time, I feel positive about the infrastructure behind our fantastic community and can now start looking to future growth rather than existing niggles.
Hopefully this thread will help with your choices - we spent a LOT of time evaluating and trying hosts.