DIY outboard passive subs

I've been intrigued with the idea of running one or two smaller box passive subs off the plate amp in my KLH 120W machine.

First, anybody here every tried running slaved passives off of this or any other plate amp from a main sub? Does this mess up the main sub? I know this is feasible, but is it wise???

Second, I'm really searching for small box sizes, but know that I need an 8" to 10" driver in these. I've read the Linkwitz page and he has a nice DIY design there that fits the bill in the Pluto sub plans, but these take some kind of Linkwitz sub circuitry. I'm not that good at building as to take on that part of the project. Is there an alternative design that keeps to the size limits I'm suggesting here for WAF?

Third, if the main sub works and I add only one or two slaves, would an 8" driver in those make sub integration harder or not? Would an 8" make any sense as the second sub?

Yes, lots of questions.... Hope somebody has the actual knowledge out there to share!

Thanks.