wiring setup thoughts

The fuses on the dist. block really depends on how long your wire run from the block to the amp is.  The 4ga wire is not rated to handle the 150A that the 1/0 wire is, so if you had a short between the dist. block and the amp, you've got the possiblility of catastrophe.   The fuses on the amp do nothing to protect the wire. 

I'd personally feel much better spending $30 or so on a good fused dist. block and not having to worry about that sort of thing. 

I would use a max of 80A to each amp.  You can use a lower rated fuse for your 4ch amp since it's not going to draw more than 40A.

I too would feel better with a fused distribution block.  However, your current setup isn't too bad because you only have a 150A fuse up front.  A direct short would blow the 150A fuse way before the 8ga wire heated up enough to catch fire.  The only safety issue you have now is if for some reason the 8ga wire started to pull 125A for a long period of time.  This is very unlikely in a car environment - a typical issue is a direct short to ground.

The bigger the fuse up front the more of a safety issue you have by not fusing the smaller wires in the back.  Some people would say you need a 350A fuse up front because that's the max the wire can handle.  If you put a 350A fuse in there you would have a huge problem with the unfused 8ga.

I always liked having a fused distribution block that used the glass fuses.  They are cheap and if you have to work on an amp or something and you accidently short something out and blow a fuse the glass fuses are much cheaper and easier to replace then a name brand wafer fuse under the hood.  It also makes it easier to pull fuses so you don't short anything out.

Thats a good rule of thumb to go by.

The only other thing to consider though is the fact that the SoundStream amp won't ever pull close to 40A.  Amps are fused for their max power and unless you start running subs off the SoundStream amp I would be surprised if it ever pulled more then 20A, let alone close to 40.

I would guess you could get by with a 100A fuse, but the 150A fuse isn't hurting you in any way.