So it sounds like a matter of how loud do I want it.
But tell me this: does the 4B *sound* better than the 3B at normal volume? What would I be gaining other than power, if anything? I've been running the 3B, but I never tried to really crank it, as it seemed...unnecessary. OR maybe I was afraid of blowing something. I'm basically unaware of the limits of my stuff, and not inclined to find out the hard way. Anyway I've never noticed any clipping at what I would consider fairly high levels; maybe I'm not as deaf as some. My imagination suggests the potential for more bass rez, or more effortless highs, but...whatever.
