The HT210 is large, heavy, very deeply voiced in the low end, polite mids, and a very hi fi top end. So, in a way, it kind of lines up with the classic SWR tone. However, it is always a guess regarding how one very differently voiced cab will behave with another cab. It will definitely add more real low end than you are getting with that particular 115, but it will do so at the expense of efficiency, which means your rig might not be quite as lound.
The HS210 is much smaller (thinner front to back), much more midrange focused, and higher efficiency (louder per watt). That too would be a different voicing than the internal 210, but if your primary goal is more low end, the HT210 would probably be the thing.