Subwoofer Isolation Transformer/Ground Loop Hum Eliminator, Blue Jeans Cable Brand, Made in USA https://a.co/d/4TxgvMb

You may find the bass distorts when using an isolator due to the transformer saturating. If so, change your gain structure by turning down the volume of the sub output of the receiver and turning up the volume of the 400.

You may not have a problem at all if the transformer is designed to saturate at some voltage higher than the sub out (say 10 Volts as most receivers never go above 7 Volts on the sub out).

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated