As already stated, don't use automotive batteries indoors unless there is adequate ventilation. They give off gasses which can ignite and/or explode.
True, but I think it is important to distinguish between older automotive batteries and the relatively recent sealed varieties. The specific batteries discussed in this forum so far have been absorbent glass mat designs. These are sealed and only emit gasses (ie, volatile acid) when under extreme use (when the pressure inside the battery reaches a certain level). This is a fail-safe to prevent explosions - and I don't see the valves openning all to frequently for applications that demand smaller constant currents such as the one discussed. Furthermore, most of the configurations discussed have been in the neighborhood of 10Ah. These are relatively small compared to automotive-size batteries and should limit accumilation of any dangerous levels of gasses to negligible. These batteries are used all the time in in-door UPS applications (ie, alarm systems, computer battery backups, etc).
Maybe cramming the batteries in an air-tight enclosure might be somewhat dangerous, other than that, four the purpose of driving a virtue amp, I would think most modern sealed AGM designs in the sizes we have been looking at (about 10 Ah) should be quite safe.