Rather than putting your good third tweeter at risk I would take a "bad" tweeter and sub it in the good third speaker and see how it plays at low volumes. That would tell you if its a bad speaker or the internal electronics. I also suspect that the failure was related to something the 2150 fed to the speakers. Have you tried the 2150 on another set of speakers at low volumes just to make sure it is not failed. An internal problem with the 2150 has to be ruled out but the way you feed the 2150 through your TV from the xbox from the computer has to present some possible failure scenarios since the source level is somewhat dependent on the TV volume. The TV audio system could be the weak link. Is it possible it went into overdrive and caused the 2150 to wreck your tweeters.

I did find some info on your Luxman that may shine some light on why it appears to play louder with less effort that the 2150. The link is here:
http://www.hifi-classic.net/review/luxman-r-115-419.html


"The dual power amplifiers can deliver high currents. Although rated for only 70 watts continuous output into 8 ohms (from 20 to 20,000 Hz with no more than 0.03 percent total harmonic distortion), they are also rated to drive 2-ohm loads at up to 270 watts per channel for short periods. The large power transformer and filter capacitance give the receiver a 3-dB dynamic headroom over its rated power output into 8 ohms."