There are a wide variety of speakers that will sound good with the Brennan.
It might be worth pointing out that Bluetooth and WiFi speakers are not the same. Sonos uses a proprietary WiFi signal; it is not Bluetooth. On the other hand, Bluetooth speakers are not WiFi either.
Bluetooth speakers are generally regarded as having less high fidelity than wired speakers. WiFi speakers can often sound as good as wired speakers. Bluetooth uses a compressed signal, so fidelity can suffer. But many people don't notice or mind.
Also, you should be aware that Brennan does not support Bluetooth output. They found it difficult to make it work in every situation, so stopped supporting it. However, it has been found to work with many Bluetooth speakers, just not with all, and not guaranteed.