Not many folks run 681 Solo's, they're out of the game these days but those who own them know which saw wins the P:W ratio debate...
Just looked it up http://www.baileysonline.com/shop.axd/ProductDetails?edp_no=37338
Pretty impressive specs
Manufacturer claims:
Solo 681: 6.7kg dry PHO, 4.8kw = 0.72kw/kg
Dolmar 7910: 6.7kg dry PHO, 4.3kw = 0.64kw/kg
Stihl 661: 7.5kg dry PHO, 5.4kw = 0.72kw/kg
I've been working on a comparison table for this type of stuff - not finished, but I have attached what i have so far. Red numbers are estimates/unofficial.