You cannot run the MB-8 in low pass. The MB-8 are a mid bass driver, not a subwoofer. So you want to run them on high pass only. Or full range. High pass crossed over around 50 to 80 HZ will get you the low end you are after. They are designed to roll off naturally on the high end where the horns of the 60 or 80's really shine.
You do not want a subwoofer on the tower. Bass is Omni directional. It will not have anything to load off of on the tower. So you would not gain anything except probably more perceived bass right near them. Bass travels distances different than high frequencies. So anything past 10 feet or more, with that sized driver, you would not even notice. Which is why you only want mid bass and upper frequencies.
We built the MB-8 after many requests for a wake surf type system and party cove system. Many wanted something that got loud but was not interested in something as far as 80 feet back, they wanted something with a ton of punch closer in. You can still hear the addition of the MB-8 for sure at 80 feet back as the upper mid bass and mid range will get there. But they are designed as an add on speaker for those wanting this type of set up. Since there is no horn in them, they cannot be compared to a 60 or 80 as they are not designed that way. They are an add on to a 60 or 80 for those that want a loud system for surfing and party cove.
So looks like you would need an additional amp. Using what you have now as far as the 220 x2 would work for the MB-8 or 80's and adding something like the SYN-2 at 200x2 to the other set. I also like to have the MB-8 on their own channels when paired with the 60 or 80 so you can cross them over a bit lower and gain them differently.