I haven't heard one, so my take is speculative, but the UPA-700 seems the least competent of the UPA series. The average solid state Class A/B power amplifier (without output transformers) can usually muster at least ~ 50% to 60% more power into a 4 Ohm load than into an 8 Ohm load.
The UPA-200 (125W/200W; 20,000uF/ch) can output 60% more power. The UPA-500 (80W/120W; 8,000uF/ch) can output 50% more power. However the UPA-700 (80W/100W; 8,500uF/ch) can only output 25% more power.
Again, it's only a technical specification, but to me is an indication of power supply robustness and potential headroom, and it seems that the UPA-700 is hampered by an anemic power supply relative to having 7 channels. Not something I would want to power the woofers, anyway
Also the UPA series have dual fans. They may only come on when needed or be variable speed (?), but fans are something I've never needed in an amplifier with sufficient heat sink area.
Steve