"The LSA has a MM phono stage, yet it handled the 2.5mV Sumiko MC just fine. The AT150 is a 4 mV MM, so what gives?"
That's perfectly normal. In most cases, a HOMC can be treated as a MM when connecting it to a phono preamp.
Along with Al's suggestion, you may want to check behind your amp. The phono preamp on your LSA is not typical in that you need to use the jumpers to connect the internal phono preamp to a line level input. Since you have been most likely using aux1 for phono, you may have accidentally plugged your TT directly into it. That would easily explain the type of sound you are getting.
That's perfectly normal. In most cases, a HOMC can be treated as a MM when connecting it to a phono preamp.
Along with Al's suggestion, you may want to check behind your amp. The phono preamp on your LSA is not typical in that you need to use the jumpers to connect the internal phono preamp to a line level input. Since you have been most likely using aux1 for phono, you may have accidentally plugged your TT directly into it. That would easily explain the type of sound you are getting.