I would guess that you would get plenty of gain for quieter acoustic gigs using a 9100 and going line in on your recorder. Going mic in is noisier than line in generally speaking, and using a pre like the 9100 allows you to raise the gain level while keeping the noise floor pretty low at the same time. But it seems that you main question is whether or not adding a battery box but leaving everything else the same will improve the sound, the answer to which should be no it won't. It won't lower the noise floor or add any improvement in sonic quality. If your tapes don't have much noise/hiss at the shows you normally attend, then you could just keep doing what you're doing since you sound satisfied with the results with your current gear. A pre will give you more flexibility in the future though, so you can tape louder shows with less risk of inadequately powered mics.