The XP-66 (no letter) appeared around 1968.
12" high-compliance free-piston woofer, 5 lb magnet, 30 Hz, butyl-impregnated half-roll surround. 6" mid, sealed in its own portion of the cabinet. Overall freq. response: 30 Hz to 19 kHz. Crossover at 400 Hz and 1000 Hz, 12 dB slope. 8 ohm. 40 lbs. Walnut veneer.
XP-66B, around 1971. 12" woofer, magnet now listed at 2 lbs, freq. resp. to 32 Hz. Still has sealed 5"mid, wide-disperson 3" cone tweeter. Overall freq. resp. 32 Hz to 20 kHz at 8 ohms. Crossover at 500 Hz and 1000 Hz. MSRP $99.95 in 1971. Equivalent to $475 each in 2005 dollars. 40 lbs. Walnut veneer.
XP-66C, around 1973. Overall freq. resp. 32 Hz. to 20 kHz. Weight down to 33 lbs.