Hi Brett!
You can replace the original SB-200 bass amp speaker with a decent 100 or 200 watt rms - 4ohm speaker. The information stamped on the original speaker itself is what I would use to find a replacement.
Another helpful hint is if the owners manual states what the minimum speaker ohm its rated at. For example, my Sunn Coliseum 300 bass amp head will dish out 300 watts at 2 ohms, 200 watts at 4 ohms and 120 watts at 8 ohms. What this means is that I can hook up one or more speakers as long as the total ohms is not less that 2 ohms. But, I will only get 120 watts with a 8 ohm speaker, 200 watts with a 4 ohm speaker, etc...
Hope this helps!
djg.