What to pair with stacked Large Advents

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.

Stacked Advents today, Why?

The advents are on the A channel in series. The B channel is open.

Stacked Advents are usually wired parallel, and those who have tried both say parallel sounds much better.

Would you mind describing your connections? For one of the channels wiring, take a look here,

series vs parallel

Did you connect + and - of one speaker to the + and - of the other speaker? If so, that is parallel.

If your wiring is series, you may want to try parallel. If parallel, that will be a 4 ohm load, and you should not connect any other speakers.