For ages we would go by the rule that you want to get to 102dB or so of output, so that would land you on a speaker that's about 93dB at 8 ohms/2.83V. Then we had a long stretch where people were using 87dB sensitive Fostex based speakers with our 3.5W Stereomour integrated with pretty good satisfaction, so that previous recommendation went out the window a little bit (despite being based on very sound principles).
The other considerations for loudspeakers would be the impedance curve of the speakers themselves (observing whether there are generous low dips that would cause one to wire the amp for a lower impedance speaker), how large your room is, and what you listen to.
Having said all that, you can get good results from a wide range of pretty common and easily found speakers. If you post the model number of your Bose speakers, we can look into how suitable those might be, but there are decent speaker kits and used speakers floating around for less than $200 that can work well, and our Pipette speaker will cooperate nicely with a 300B amp if you have a spare sub available to pair with it.
