A good economical woofer section could include the Dayton 10" DVC sub that's on sale for $25. You could drive it with the 75 watt MCM plate amp that's now on sale for $40. The Dayton DVC sounds really good in a 1.5 cu ft sealed enclosure, so everything would fit fine in the original One Pi Tower enclosure with the mid, tweeter section sealed off from the woofer section just below the Beta 8.

Here's another idea that would be helpful if you were driving this with flea power SET amps like my Paramours. You could drive the subs' plate amps directly from the line output of the preamp and insert a passive high pass filter at each SET amp's input to roll off it's output below 100hz. Relieving the amps of low bass duty would enhance their ability to drive the Beta 8's.

Now I'm confused about which version of the enhanced One Pi tower I should build:)

I would run the low woofer up to 150Hz or 200Hz, so that it can smooth the floor bounce modes that would exist for the upper woofer were it to be operated alone. As I said, I would position it no further than 1/4λ below the midwoofer at the crossover point. Tweeter crossover is about a decade above low woofer cutoff, around 2kHz. The tweeter should be placed just above the midwoofer, less than 1/4λ away at its crossover point. Each of these crossovers can be made first-order, because none of the drivers is particularly sensitive to over-excursion, and each driver is to be placed within 1/4λ of the adjacent driver at the crossover point.

Somebody has asked about buying the existing One Pi tower enclosures, and if he does I'll be left with a pair of one Pi kits with no enclosure, and that will provide the motivation to buy another pair of Alpha 8's and get started on the dual woofer towers. In the meantime if anybody else in the Houston area wants the tower enclosures they're $100 with internal damping material, bi wire spkr terminals and ports installed (and your word you'll order the One Pi kits from Wayne).