You could throw some resistors inline, but it's not recommended as there are several caveats. First, they will get hot as Heck. Second, resistors don't just reduce voltage- it's dependant on the amperage or something like that. I can't really explain it well, but basically if you change the amperage you are drawing, you change the voltage that gets put out. Third, putting resistors in parallel will allow you to pull more power through without them exploding, but it will reduce the total resistance. I believe it is R1xR2/R1+R2 for two resistors, but I'm probably wrong.
Also, I do not recommend powering anything off AAs, ever. The problem is that rechargables are 1.2V, and alkalines are 1.5V. If you plan to use rechargables, alkalines will most likely fry stuff. If you plan on using alkalines, rechargables won't power it. Six alkalines provides 9V, but six rechargables puts out only 7.2V. Seven will get you a better 8.4V. So unless you have a good flexible power regulation scheme, I don't recommend AAs. If you do use them, clearly label which type to use. Maybe provide a nice switch to bypass one cell? Maybe some sort of autoswitch?
Now I feel dumb, I didn't know that rechargeables were only 1.2V. My 6 AAs are rechargeable, but you're telling me now that 7.2V is too few? And from what I understand, you're saying I can't mix it up a little with say, 5 rechargeables and 1 alkaline for a total of 7.5V? I do have two 3.7V Li-Pos with appropriate charger and prot circuit. I couldn't ever get them to work (because I'm incompetent), but it seems that they are my best option other than buying any more batteries.
Man, batteries are my most hated part of portablizing, bar none.