Hi Beehive,
This is a good question. So, as I understand it, you originally had a city-specific domain name when you were a single location business. You now have six locations and have moved to a purely branded domain rather than a city-specific one and this site has a unique page for each of your locations (which is good - a best practice - provided the content is unique on each page).
I would not recommend putting all six city names in your homepage title tag or in the title tag of any other page. Once a business expands like yours has, you've got to come up with a different strategy than the one you were using for a single location.
Questions:
Did you 301 redirect the old domain to the new one?
Did you update all of your citations to reflect the new domain name? Realize this will take time to go into effect. This is very important.
Suggestions:
You might like to read this post about local landing pages: http://moz.com/blog/local-landing-pages-guide
Study the part about Business Model III in the above guide, if you've not already read it. It applies to your business model.
A multi-location local business that I think is doing a good job with their Local SEO is REI.com. Check out their location landing pages.
You must now start building the authority for each of the 5 new locations, with content development, citation building, review acquisition, social outreach, etc.
I hope some of these thoughts will be helpful to you.