That puts things in a better perspective.
If you use a lineout for the sub, just about any “decent” Class D amp will do. Just make sure to get one with enough power. That’ll be determined by your choice for the sub.
To the sub itself. I started running through the list on PE last night. There’s quite a few there, and that’s only one place. If you start looking at other places, it could take a while to narrow it down. I’m looking for something that has deep response without needing a huge enclosure. With your max height of 22″, I’m thinking that up to between 2-1/2 to 3 CuFt would be about as big as you’d want, preferably as small as possible. Most calcs I run bring in either a huge box (6 or more CuFt) or a less than stellar bottom end. But there’s still quite a few left to go through in the 10″-15″ range.
Would you be averse to using multiple smaller subs? Say (4) 6″ to 8″ drivers in 1 box? As an old friend of the family who is in the PA / Concert sound system business once said, “Give me voice coils. I can make 4- 10″ woofers play louder and hit harder and deeper than 1- 15″ .” And for every extra cone you add to it, you get a 3dB boost. So if one particular driver is 88 dB, 4 of them would be about 97 dB.
Just some thoughts.
Nick, I still have some end table speakers my late brother-in-law (and best friend) made back in the early 80’s. They will be going to my oldest once he buys a house. He was looking at some even older side tables that had 8″ drivers in them from ?the 50’s to 60’s? at his local Goodwill. Would have been interesting to hear them.