What to pair with stacked Large Advents

Since you have Advents, the Baby Advent should be a good match.

Not a bad idea. Never heard them but probably a good compliment to the large ones. Thanks!

Refurbished Small Advents would be a much closer tonal match (than the Baby), plus they will perform better @ higher SPL's.

Owned stacked Large Advents in the mid 70's and when ran as single pairs preferred the Small Advent (over a pair of the Large).

Never tried stacked Small's (wonder how they would sound).

I personally wouldn't add a pair of 'rear' speakers. I don't think the Luxamn could handle it, it would take away from the power needed to push the stacks. I had fun refurbishing two pairs of Advents myself, the stacked pairs can sound almost concert level.

did you consider diy upgrades to your luxman by going for lager filter caps? that may somehow compensate lack of current you need for so many speakers.