Those are two different animals. The Martin Logan is probably more comparable to the SB12 than the Plus/2. The Plus/2 is a monster made for high output. It's also the size of an end table. On the plus side, it comes in more than one finish than black, and will have more than enough headroom for your basement.
If it's within your budget and you have the space, my choice would be plus/2. If you want something smaller, nothing wrong with going with what you already have and like. You may want to consider the SB12 too, given it's a closer match to what you're considering (sealed, smaller, 425W). The plus/2 will fill the space better though.
Also worth noting, the price of the plus/2 goes up $100 on Jan 2. So take your time and decide quickly