It took me quite some time to appreciate the TM Alto kalimba. The expressiveness of the wah-wah that one can achieve by covering and uncovering the sound holes of the regular Alto kalimba is not an option with the TM Alto kalimba, as it is mounted on a flat board. The wah wah is integral to my technique on kalimba. But now I see the TM Alto as a whole different instrument.
First, it looks different. The shape of this kalimba resembles the body of a traditional 8-note karimba from Africa. The illustration below, from a Hugh Tracey field recording CD, shows one of these ancient kalimbas, which is played in contact with a small resonant gourd. The shape of the TM kalimba also closely resembles AMI's trademark, which Hugh Tracey designed before he started making kalimbas in the 1950s.
Second, it feels different. The narrow flared wood fits in your hands differently, and the vibrations last longer and you can really feel the hum of the notes in your hands.
Third, it sounds different. Box kalimbas have a strong resonance in the midrange, which changes to a weaker resonance at lower frequencies when you cover the sound hole. The TM Alto doesn't have this kind of profile to its resonance. The EQ curve of this instrument is much more level than that of the box Alto, i.e., each note has a similar volume.
Last, the curved lower edge of the TM Alto kalimba allows this instrument to fit nicely into a gourd or a bowl, which acts as a resonator. This totally changes the sound, makes it three or four times louder, and gives it a bit of a buzz. This is an authentic African sound.