If I could offer my own opinion here, that might help save some time and/or money. I've used a variety of tactile units, ranging from the exciters, aura bass shakers, BK mini's, Earthquake mini unit (MQB-1) and now I've just ordered a BK Concert large unit to add to the rig, just to give a little background.
Go straight for a decent sized tactile unit, with a good 2 or 4 channel amp straight off the bat. The ease of use of mounting and running a single large unit, from a decent sized amp is way better than mucking around with 4 little exciters, little amps and a bunch of cables running all over the place. You should be able to add a decent amount of effects even to a single unit since there is a bunch of overhead in the amount of power it can handle before it reaches its limits and the actual sensation you'll get should be awesome. I say should, since I have yet to try my large unit as it has not arrived.
I've made the mistake of messing around with little units, trying to get a good result from a budget oriented build. Since sim racing is nearly all I do now, it makes sense to spend the money once and enjoy the best tactile that you can. So, if you actually do have the money and the intention to run a decent tactile profile, definitely grab a single BK LFE and something like an NX1000D. That amp should be more than enough to power the unit, you have DSP control to get the most out of its low end and even a potential bridge mode should you find that a single channel does not offer enough power. You could then, if you found the need, add another unit to that amp, either left / right or front / rear config, for a more complete feedback.
You'd be looking at a decent initial outlay, but it almost guarantees that you don't really need to buy anything else from this point. If you're only looking at adding a decent level of tactile, without going overboard, you just mount the unit under your seat and you're done. It's clean, efficient and has a tonne of power that you're sure to make the best use of when compared to those little exciter models. I ran them with 2 x BK Mini LFE's and when I disabled the LFE's, even mounted directly onto the seat the little exciters could barely be felt.
So I know intend to run 1 x BK concert on the seat from the NX1000D, and keep the 2 x BK Mini's running from my little SMSL 2 channel amp which has been more than enough for these units. Eventually I think I'll end up with all units mounted directly on the seat, but at this stage I have 2 units on the seat (one being the Earthquake unit) and 1 Mini LFE under the pedals. Might remove the pedal one and run them both in L/R config rather than F/R.
Just my 2 cents since there seems to be shortages of the units you're seeking. It might give something to think about.
If I were to know that I'd end up with the rig I do now, I'd definitely skip all the middle ground and just get one large LFE unit with the NX1000D amp straight off. See how that suited me and most likely call it a day.
