I had some really old Velodynes- one was an 18" bottom firing coffin with a separate control box, another circa the mid 90's - was a front facing 15". While both were servo woofers, neither had any DSP. I used them for home theatre back in the day, not for hi-fi. Both worked when I gave them away- I was moving and didn't see the need to move them.
Replacing the plate amp from a third party won't necessarily deal w/ the servo part. And shipping costs are a huge factor these days. I bought a bunch of Rythmiks, very cost effective, that I use in my main system and in my downsized "home theatre" (over which I could play music if I wanted). Both use DSP. Really helps integrate the subs to the room.