With the SYN-4. You would wire on pair on chs 1-2 and two pairs on chs 3-4. The SYN-4 does 125x4 at 4 ohms and 200x4 at 2 ohms. So you wind up with 125 per speaker on the single pair and 100 per speaker on the two pair running in parallel.
With the 6450. that amp is 45x6. So you would just wire them up to their own chs. however you would be 45 watts so over half of what you can do on a syn-4.
But since you have some needs on the tower. And want to do it with the least amount of amps. Here is how I would run it.
Two SYN-6 amps. Run one on the XS-650 in boats. The SYN-6 is a hybrid that has a smaller 4 ch section and larger 2 ch section.
At 4 ohms it does 60x4 and 155x2. Bridged it will do 200x2 and 600x1. Chs 5-6 are high pass or low pass. Which is a very unique feature allowing you to power a sub or subs or tower speakers
So in your set up. You would wire one SYN-6 on the XS-650 off chs 1-4. So one pair would be getting 60x2 off chs 1-2 and the other two pairs in 2 ohm stereo off chs 3-4 for 105x2 so 52.5 per speaker. Then bridge chs 5-6 on the single 485 for 600x1.
Take the second SYN-6 and run it in 4 ch mode. So it will be 200x2 and 155x2. 200x2 on chs 1-4 bridged to the MB-8 and 155x2 off chs 5-6 stereo to the PRO 60's.
This would give you awesome power on the tower with a bit more on the in boats versus using the 6450. Not as much as using a dedicated amp for the in boats. But the best solution in terms of amount of amps to run what you need.
Other option would be SYN-4 on the in boats. SYN-2 bridge on the 485 and SYN-6 on the Double UP or SYN-2 on the Double UP.
You can download this step by step guide on tuning a system and setting it up using the WS-420. If you dont have the 420, you just skip a couple steps. This is a really good "setting a system by ear" approach. http://www.wetsounds.com/media/produ...stallation.pdf