Boston Acoustics XB6 Subwoofer to Airport Express

Airport Express using the 3.5mm jack to the single RCA Input (mono Cable)

Just to be clear.. I am guessing having got the standard 3.5mm stereo plug to mono RCA you plugged it into the RCA input of the Subwoofer and no joy. You get music via the HomePods but nothing out the sub.

If that is correct, the issue could be the iPhone rather than express. Try sending the music (with loads of bass) only to the express.

From the manual I got horrible pictures.. so excuse the screenshot.. starting from the bottom use the top RCA input not the bottom one.. You want the use crossover control, not bypass.

Set the crossover to 150Hz.

Polarity doesn't matter, but should be set to 0 in most cases.

The main power switch should be on and main volume control set to about 11 position. (not in the picture)

Power mode should be on. Auto would be better but for now you want to force the Sub to work.

If that doesn't work test the sub on some other device just to check it is working.. headphone output on any TV, line out on a computer, etc anything you like.. you should get something.. remember though this is only the very low end freq.

You already tested the Express with headphones and it worked so that is good.

Not Clear this can be done?? as I don't understand how to connect the Airport Express as an Input into the Amplifier?? Additional Cabling??

This should work.. The wiring is not hard.

If no luck I think your phone is not actually sending the music to both the HomePod/s and the express.

Tell us how the test goes.

Might be wise to first confirm that the AirPort Express is working correctly.

Temporarily, plug a pair of stereo headphones directly into the 3.5 mm jack on the AirPort Express.

Can you hear music that way?

If yes, you know the AirPort Express is functioning correctly and the problem is in your connecting cable.

If no, then we'll have to work to get the Express going first using the headphones.....before moving on to try to troubleshoot other potential issues.

Possibly bad cable? Yes

(everything is on and can hear humming That is a bad ground connection somewhere in the cable.

sub not amplified? If the sub is not amplified, you won't be able to control the volume of the sub relative to the mid range and highs.

You should be able to keep the volume of the AirPort Express relatively low and raise the volume control on the Express if you want to raise the sound level of the sub. But, if the volume is already high on the Express, and you don't have enough volume on the sub, not much that you can do in that case unless you have separate control over the sub.

sub not amplified?

The express puts out line level.. not suitable for subwoofer directly. You must put it through the onboard amplifier.

I have tried moving the Line Level In - to the use Crossover Control

Not sure this can work?

It should. The normal setup with a modern Amp that has sub out is to plug that into the sub using the crossover but it is effectively already filtered by the main amplifier...

You do not want or need full audio frequencies to go to the sub. You should be using the line in with sub filter control.

Your speakers are powered type and should work fine.

Remember with ordinary music levels your sub puts out only signals below 150Hz with the control right to the max.. in normal music all you should get is very bottom bass.