I'm using a Rel Storm III with my Mani-2s - just to get that last little bit. It's arguably overkill, and in fact for a lot of music the sub isn't even noticeable (ie I can turn it off and not miss it).
Given you already have the Manis, why not buy a sub and pair it with them first? That is, why do you think you have to trade in the Mani-2s if all you seek is deeper bass?
OTOH, one thing I noticed is it's easier to integrate a sub if the crossover is at a little higher point. I had Totem Sttafs prior to the Manis. It took a few nanoseconds to get the Sub set up with the Sttafs (x-over at 38Hz), but a fair amount of effort to do so with the Manis(x-over at 30Hz). I guess the other reason may be a little bit of cash you may net in the exchange, but I still don't think I'd do it if it were me.
Given you already have the Manis, why not buy a sub and pair it with them first? That is, why do you think you have to trade in the Mani-2s if all you seek is deeper bass?
OTOH, one thing I noticed is it's easier to integrate a sub if the crossover is at a little higher point. I had Totem Sttafs prior to the Manis. It took a few nanoseconds to get the Sub set up with the Sttafs (x-over at 38Hz), but a fair amount of effort to do so with the Manis(x-over at 30Hz). I guess the other reason may be a little bit of cash you may net in the exchange, but I still don't think I'd do it if it were me.