If you have a Bryston 4, you're correct in thinking that it will be able to handle almost any small speaker with ease. But that doesn't solve the problem. There's 2 things you need to consider.Regardless of what happens with bass, all speakers are different. Some may be able to play loud enough for you, while others can't. It has to be taken on a case by case basis, and not generalize.
More importantly, If you have a small speaker, the sub will almost certainty not be able bridge the gap between sub frequencies and the lowest frequencies the small speakers put out. Using an xover to cut out the lowest frequencies in the small speaker is only going to make the problem worse. I think this is where the confusion is coming from. Subwoofers are not woofers. They are not meant to play low frequencies, they're meant to play sub frequencies. The right way to do this would be to use a sub with full range speakers. That way, everything is doing what they were designed to do.
More importantly, If you have a small speaker, the sub will almost certainty not be able bridge the gap between sub frequencies and the lowest frequencies the small speakers put out. Using an xover to cut out the lowest frequencies in the small speaker is only going to make the problem worse. I think this is where the confusion is coming from. Subwoofers are not woofers. They are not meant to play low frequencies, they're meant to play sub frequencies. The right way to do this would be to use a sub with full range speakers. That way, everything is doing what they were designed to do.