You could invest in USB audio contraptions rather than Firewire, but that obviously leaves behind all the money you spent on your Firewire gear. Most new tablets have USB 3.0, so there's plenty of bandwidth at least.

In terms of new tablets, there are several 18+" tablet with desktop docking stations and internal batteries long enough to be usable for your gigs. You might like the Sony Tap 21, the Dell XPS 18, or the Asus P1801 (and there's probably more where they came from).

Have you considered a standalone audio recorder of some sort? You might want to separate out your audio recording from your sheet music display for a variety of reasons.