Hi
I got a jumbo JF30-12 maple, arched back solid top on Ebay, I think made '91 Westerly Factory. Westerly factory was my most important requirement, they have never sounded quite the same since. I got lucky, The one I got I think had basically never been played. Original strings, original everything. Fundamentally the JF-30 is the 'F-512' with less bling.
Fantastic guitar. I believe if it's an arched back it may be perforce laminated but not to be cheap, but the way to get the arched back, and they are really good wood pieces used for this, not cheap plywood. I also have a Rosewood 1980 F50 6-string. I had heard the Westerly Maple 12 string and always wanted that specifically for the specific sound which has a lot of great resonance and sound on the mid and high and is still clear enough in the Bass, but not too 'Bass-y'. It does sound a little bit different than a Rosewood jumbo 12-string F-512 of the same shape. It is 'brighter'. At the same time those also would have been arched back also. My F50 (Rosewood) is arched back. So if you don't like the Maple tone, you can look for the Rosewood one, the model # F-512 on Ebay. They are both awesome sounding. The necks are not narrow so be prepared for that. Even the F-512 will not be 'really really Bass-y' but it may sound very slightly 'warmer'.
As far as the new guilds, they have gone through several factories since then and each time the sound has changed some so I'd have to hear the specific guitar before
buying it if it was not Westerly. There was a factory for a while making them maybe in California but they didn't sound the same for sure. Then there was a factory in Conn. that was trying hard to make good new guitars... online they seemed OK but I couldn't find any place that had the high end models in stock to try in person, so I couldn't get a good opinion.
Now they moved the factory again to California I hear, and of course China. The wood is different, build process different, even the air and humidity during curing processes can effect instruments... so for new guitars, I think I'd have to hear it to see how they are coming out now, and still I cannot find any place that has the higher end models in stock. Plus they are all two-piece backs. That is 100% sure to add more Bass. So that may move a 12-string maple into the range of Bass you are looking for if you worry the original was not Bass enough for you.