Can you force a device to choose where it connects?

Yes.. you have a poor link.. while described as good.. anything that low will be unstable.

The Express is more than likely getting worse signal again and so you really need to fix this with ethernet or Powerline adapters to the Express in a more suitable location.

I would also wonder if the wireless on the older TC is not fading. It is now over 6-7 years old if it is gen4 and worse for older one. We find wireless signal gets worse as time goes on.

There is a good chance the Express is getting poor signal from the TC and so is actually doing no better than the speakers.

The best position for the Express is half way in-between the TC and the speakers, assuming you are not using any wired connection, whether ethernet or airplay from the express.

Test the actual numbers. In airport utility you can get some idea of the link quality by hovering your mouse pointer over the word Unknown concerning connection. A popup menu will show details of the Unknown connection.

This connection is bound to give trouble.. the RSSI ie signal is far too low at -75dBm. You really want signal no less than -65dB at its worst. (remember negative numbers the larger the number the worse the signal.) Really excellent signal will be around -50 to -40dB.

Are you using this AirPort Express to extend the wireless network provided by the Time Capsule (TC) or to join it for the purposes of using AirPlay? If the latter, your Bose speakers will always connect to the network provided by the TC. For the former, there may be a way to "trick" your speakers to only connect to the Express base station, but most likely, they are connecting to the TC because it finds its signal to be stronger.

Not sure if you were aware or not that when the AirPort Express connects wirelessly to extend the network that you will lose about half of the potential speed on the network when this occurs. So, while the Express might be producing a stronger signal, it may well be a significantly slower signal with higher noise content.

Keep in mind also that the Time Capsule has a much better antenna arrangement than an AirPort Express. In the case of the Time Capsule, the antennas are located right up at the top of the "tower" while they are really buried inside the Express.

So, it is possible that the Bose speakers are actually receiving a better quality signal in terms of speed and noise from the Time Capsule than they are from the AirPort Express.

If you have the time to experiment, you might consider configuring the AirPort Express to "join" the network.....not extend it.....and the network might work better that way.