Just ordered a Laney TI15-112 Iommi combo amp, and would like to have a spare set of tubes on hand.

The manual says it takes "three 12AX7 preamp tubes, and 2 EL84 output tubes."

I've never bought or replaced tubes before. Will any tube with these specs do, or do different brands produce different sound characteristics? The manual doesn't specify a brand so I wonder if it matters.