Assuming you don't have a sound pressure meter (like the old Radio Shack one), you might download an SPL meter app for your phone and use it to measure how loud you like to listen to music. One person's definition of "loud" isn't necessarily the same as someone else's. My own definition of "loud" in my small listening room (11' by 12') translates to peaks at 90-95 dBs. With my speakers (97 dBs sensitivity), a 2A3 amp would work fine. But if my version of "loud" was, say, 105dBs, I might not be happy with 2 watts. (in which case, the Kaiju would be the better amp).
Different horses for different courses.