I've got a Ibanez 12 string with the SRT Preamp with B-Band under saddle pickup with no output. It uses 2 AA batteries which I just changed. It has both XLR and 1/4" outputs, but neither are working. There does seem to be signal going to the amp, but there is no volume output at all. I know these preamps are somewhat notorious for having this kind of issue, but I haven't found a good fix besides replacing the preamp. Any help or suggestions would be appreciated.
Before you replace the pre-amp, try connecting the pick-up directly to the amp. If you've got signal, then you can start working on the preamp. If there's no or marginal signal, check the solder connections at the pick-up.
Joshua's advice to check the pickup output is an excellent start. I keep a 1/4" jack with alligator clips attached in my tools for this purpose.
A couple of other things to check:
1. Clean the controls with Deoxit. Recently had an Epi acoustic that had intermittent low treble response (among other issues) and this cured the issue. Top mounting makes the preamp controls prone to fouling with sweat, dust, etc.
2. The circuits on these are pretty simple. Inspect the circuit card (preferably under magnification 10-20X). Look for damage to components - particularly the capacitors around the op-amp IC(s). I recently repaired a Fender Acoustic bass that had a blown op-amp and a couple of damaged capacitors. Based on the evidence in that case, I suspect it was either Static Discharge or (more likely) reverse insertion of the batteries (no polarity protection in the circuit).
Hope this helps.