The former Chief, who will turn 32 next month, ranks 12th in NFL history with 128.5 career sacks.
He has had 11 or more sacks in eight of his ten seasons, with 2004 and 2006 the only exceptions.
Along with the anticipated return to health of Von Miller, adding the Idaho State alum would put some teeth back into a pass rush that struggled mightily in 2013.
Denver's sacks fell from a league-leading 52 (tie) in 2012 to just 41 last season (13th, tie), largely due to the departure of Elvis Dumervil and Miller's six-game suspension and season-ending injury.
Signing Allen figures to cost a good deal more than keeping Dumervil would have; Minnesota paid him a whopping $14.5M last season.
The Broncos were seen as a potential suitor for Allen prior to last season's trade deadline.
A native of Dallas, TX, Allen played at least 962 snaps in each of his six seasons with Minnesota, but he dropped to a -4.0 overall grade in 2013 (from +26.9 in 2011 and +5.1 in 2012) according to PFF.
After having been franchise tagged by the Chiefs after four seasons, Allen was dealt to the Vikings in the days leading up to the 2008 Draft.
In exchange for Allen and the pick that would become Vikings center John Sullivan, KC received a handful of picks that would eventually become Branden Albert and Jamaal Charles.
Incidentally, Albert is expected to sign a $10M/year deal with Miami once free agency opens on Tuesday afternoon.