Best sounding low power (under 80watts) Class AB amp?

Ayre Acoustics AX-7E, hands down great amplifier.

Ahh sorry missed the class ab bit of your post. 

How about a Schiit Vidar? At 100 watts/ch, it is a shade more powerful than your 80 watt spec, but definitely class AB and 22 lbs and a fairly small footprint.  (In my case, I have their class A Aegir which is much lower power, but all of their amps have excellent sound quality.)

+2 On the Schiit Vidar 2 amp

Mike

https://www.schiit.com/products/vidar2

I am enjoying Quad 405 amp - has pretty and engaging sound.

Although its an old design, it is an affordable classic British offering...A little over at 100 watts per ch advertised - as you know, this depends on who is doing the measuring and how it is done, so this probably is better than 303 amp - and they offer even more powerful one. 

Its also smaller than most amps. 

Their pre amp is good too - most people match them together - better than adcom, B&K, and other ones at the price point.