I got some at a yard sale and am comparing them to M3s. The M3s are definitely clearer - but the AC600s do pretty well for a speaker so old they are not even in the archive section of the pinnacle website.

Specs (from a curteous Pinnacle employee):

AC600
Features:
Video Shielded
Continuous bevel cabinet styling
7 Year Transferable Warranty on parts & labor
Made in USA (New York)

System Type: 2-Way, 4th Order rear vented with patented Diaduct tuning

Component Drivers:
3/4" liquid cooled polyamide dome tweeter
6 1/2" fiber-cone woofer with rubber surround

Frequency Response: 40-21 kHz (+/- 3 dB)

Crossover Frequency: 4 kHz

Recommended Amplifier Power: 15-125 Watts RMS per channel

Operative Power Range: 15-75 Watts, 225 Watts Peak

Sensitivity: 91 dB @ 1 Watt / 1 Meter (2.83 Volts Pink Noise)

Nominal Impedance: 8 Ohms

M22s, QS4s, M2 center, Hsu stf-1.