So I've had BK Mini LFE's, the exciters of which I had 2 varieties, BK Concert and TST 239. Among others but those are relevant to the comparisons and conversation at hand.
I had the Mini's mounted one under and directly attached to the seat, and one under the pedals on the chassis. I had 6 exciters on the seat, 4 at the back and 2 at the sides. I could notice them a little bit, especially when the Mini's were turned OFF, but when they were running in tandem with the Mini LFE's, honestly I could barely feel them. I did A/B tests before I decided to remove them and not bother with them anymore and the simple fact for me was that they weren't worth having on. I only had left and right channels assigned to them, but the issue for me was not the ability to assign effects. It was that from my testing, they could barely make a difference with the Mini LFE, even if only one of those units was attached directly to the seat.
Now, I have a BK Concert, the large one, and a TST 239, both mounted under the seat. Not directly bolted onto it like the Mini was, but in such a way that others have shown using their custom plates bolted to the seat rails. So basically the optimum way to mount the larger units without having them smash directly onto the seat shell. I fail to see how, now with LARGER units than before, the exciters will give anything of worth against such powerful output of the Concert especially, but also the TST that shares the same hz range as the exciters just at a much larger power output. For me, I'm not just saying they are too small and worthless. I actually had them for a while combined with the Mini's. If you consider Mini's + exciters being 100% feeling, if I turned the exciters off I ended up with maybe 90% feeling. To me, that's not worth. It did help round out the feeling somewhat being all over the seat, but with the Mini being directly bolted to the seat, the whole thing was receiving tactile for the most part anyway. So if one Mini was enough to negate the performance of the exciters, what would 1 x Concert and 1 x TST do to negate them!? I doubt I'd realise they were even on at all.
So this revelation of using large units underneath and then using Mini's as the exciters, so to speak, is actually a really intriguing one. I think one that would work FAR better than the aforementioned recommended build. Honestly I did like the Mini LFE's a lot. For what they are, they are pretty good. However, the price of two of them is equal to buying 1 x Concert or BK LFE. So now I would always recommend to get one of those first and only get the mini LFE's to work as metalandwood has explained, as exciters.