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.