David,
Hopefully someone has some pics of different set ups on the monster tower. It looks like you can fit the 3-Some. But not the pair of 485's.
So you could add a single 485 now with what you have and add a pair of 60's making a 60-3-Some later or pair of 80's making a 80-3-Some.
Another option would be to pull those and do the Double Up system. It has the MB-8 with the PRO 60's. Works great on the narrow towers like that one and sounds amazing.
Of course you can do the PRO 60 pair or 2 pairs or PRO 80 pair of two pairs or PAIR of 80 and PAIR of 60's etc.....
Lots of options. All boils down to the look you prefer and the sound you are after. As in the loudest on the lake, great combo of sound quality and loudness, or simply just a bit more sound to hear while riding etc....
You can see different amp configurations here.
http://www.wetsounds.com/pages/products/SYN6.html http://www.wetsounds.com/pages/products/SYN4.html http://www.wetsounds.com/pages/products/SYN2.html Tim
Wet Sounds