JTHED,
Alkaline batteries are best used as a backup in the Firestorm, and not as the main source of batteries. Rechargeables are your best bet, and will yield the best results in your unit. I like to run both speakers all of the time on my units, and I can get days of use before they need to be recharged.
The batteries that we are using are an excellent choice for batteries. Like the Sanyo Eneloops, they are a low self discharge, high capacity NiMH battery. They are hard to beat. Normal NiMH batteries self discharge at a much higher rate than low self discharge batteries. In a nutshell, that means a fully charged set of these sitting on the shelf will last a lot longer. If you leave your caller sitting on the shelf for a week or more, the regular NiMH batteries that were fully charged could now be much less. Whereas the low self discharge batteries will last much longer on a full charge sitting on the shelf.
Hope this helps!