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:)