How, exactly, will you use the 4180 or 2180 to power the SI BM? Since these don't bridge, and even if power doubles at 2-ohm, it's still 360w, which, despite what Nick says, I think is a little underpowered.
I have a BM MKIV, and would love to use these amps. I saw BT a while ago, but always wondered how they sounded for SQ. IME, bikes generally want things LOUD, but not necessarily detailed or nuanced, since you can't hear nuances anyway.
Also, do you know if the 4180 could bridge two channels internally for 3-way operation? (the dream)
Then there's the whole too good to be true thing...