I have read that there were three versions of the Thunderbolt, with some significant differences.
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There was a used Thunderbolt 3 for sale on the Audiogon site a few months ago, but I believe it was sold. I'd suggest you place a wanted ad on the site and see what happens. I had wanted to find one, but got a JL sub after months of waiting with none showing up on Audiogon. The JL works fine with my Nolas, so I didn't pursue the Thunderbolt when it appeared. I'd suggest you consider that or a REL subwoofer if you can't find a Thunderbolt. Good luck.
Don't see Thunderbolts come up for sale very often but I've been using REL subs for quite a few years with my Alon's/Nola's with terrific results.
You may also want to consider doing the woofer low-pass x-over upgrade on your Alon 1's. I just did the upgrade last night on my Alon 1's and it is a very worthwhile/performance upgrade and that observation is just a day into playing them.