S+R. I agree with what you say about the bass DI. I don't know about Nick's bass rig but but most onboard bass amp DI outs have a pre/post switch and I always set it to "pre", sometimes while the bass player is not looking. Then I get him to play the amp on stage and he fiddles with his settings until he has "his" stage sound, and then I try to get it through the PA. That way if I need to alter it to get it right in the mix I can.

If it becomes certain that the Phonics has no gain and no clip light then bearing in mind that WLD is right and you can get big peaks out of a guitar, particularly if you are a chord player but also if you are one of the pickers who plucks the string away from the guitar and then lets it snap back towards the body test (perhaps to a lesser extent if you pluck sideways rather than using the classical technique of pushing the string down and then letting it spring up) level, then I suggest sitting down somewhere nice and quiet with the PA and both the acoustic guitars. Start with the one that has got a preamp and therefore (I imagine) a volume control on the guitar. Set the channel fader on that channel about where you had it for that guitar. Now turn the guitar up until you hear it start to distort. Just as s test, operate the "pad" switch. This will quieten the guitar by 20 Db. You will need to turn up the main out fader to hear the guitar properly now. The distortion will go away - or it should. If it does then you are probably going to need to turn the guitar down on the guitar if you want to take the pad switch off again. Alternatively you could turn the guitar up on the guitar and leave the pad switch switched, if there is enough "welly" in the onboard guitar preamp.

If you have a guitar preamp, always make sure it has a new battery in. At one beer/folk festival a couple of years back near me the soundman was going berserk trying to get a decent sound out of one particular blonde's guitar (a Seagull). It was so bad I went away and came back with an old valve PA head (huge input impedance) to run that guitar through, but still no improvement.   Eventually the battery in her preamp flattened so much there was no sound. The soundman said "have you got a new battery for your preamp?" and she said "what preamp?". Obviously the battery had been in the guitar from new!

Anyway, by this route you should be able to sort that guitar. It might be worth trying a compressor (or a separate guitar preamp turned down) if you can borrow them to try.

Now back to your guitar. Is it louder than the one with the preamp? If so try the pad switch (or a separate guitar preamp turned down - or even an electric guitarist's volume pedal). If you have, as we expect, a magnetick pickup, try it out in a shop with an acoustic emulator (designed to enable electric guitars to sound like an acoustic). Or try the Fishman Rare Earth slot-in pickup (expensive) if you want to stay with a slot-in. I hate them but some people love them.

If your guitar is quieter than the one with a preamp, it isn't front end overload distortion, so only two options are left: it's your guitar/pickup/picking sound as such, or the pickup in your guitar needs a wholly exceptionally high impedance at the amp. This is unlikely if your pickup is a magnetic. But if it is the case, then you will need a proper preamp for it or change the pickup.

Try your guitar through an "acoustic amp" if you can borrow one. Many people like the Trace Elliot but it is expensive and frankly I think it is a bit scratchy. The Marshall acoustic amp is a lot cheaper and has been very well reviewed. If you know an old man with an old valve PA head and a speaker, try that. If those "sweeten" the sound of your guitar then you could acquire such an amp, and either DI it (there may be a slave out on the old valve PA head) or mic it, but it will almost certainly be cheaper to change the pickup in your guitar or change the whole guitar!.

If you can get a Yamaha with all the gubbins in for the price mentioned above, it maybe worth trying at least as a comparator to see what difference it makes. Or borrow a Takamine to try. Plugged they do not sound like an acoustic to my ears, but they are very popular as pluggable acoustics. You can probably get a Taylor off ebay for about for not much over £500 - but again they don't sound hugely like guitars to my ears, plugged or unplugged, although the lad who bought the last one I sound (for a mate - it was a 512CE) loved the plugged sound and bought it expressly to play in pubs for money.

But try the amp setup and the other guitar first. I reckon the problem is likely to be your pickup.