Having designed speakers on the S-30, let me just toss out some observations...
The S-30 has an unusually high output impedance, or low damping factor. It will put out more power into an impedance peak than into an impedance dip, which is just the opposite of a solid state amp. So if the Songtower was "voiced" on a solid state amp, its voicing probably would not benefit from being paired with the S-30. The Zero will lessen the magnitude of these effects, but not the general trend. One would have to eyeball the impedance curve and frequency response curve (presumably generated using a solid state amp) of the Songtower simultaneously to judge whether an amp like the S-30 would be a good match.
As a general rule of thumb that SET guys came up with, you want the theoretical 1 watt SPL maximum to be at least 102 dB. So if your speakers are 92 dB efficient, you want at least 10 watts. This assumes SET or OTL amps which are benign when driven into mild clipping; with solid state amps, you'd proabably want twice as much power. Also if you have a big room, or an overdamped/open floorplan room, or listen pretty loud, your target calculated SPL needs to be higher.
