In the OP's case it is (no more, and no less) than 8 [ohms] divided by the output impedance of the amplifier in question -- the output impedance is thus, nominally, 0.1 ohms.
A DF "spec" is next to meaningless as far as I am concerned, at least in terms of its "absolute value", since it would only be accurate for a purely resistive 8 ohm load. Most loudspeakers (virtually all loudspeakers) are not resistive loads, but are instead complex reactive loads (with frequency dependent changes in capacitance and inductance).
An amplifer with a "low" output impedance will "feel" its load less than one with a higher output impedance.
The problem, howeverl is that the cure can be is worse than the disease. Especially in the spec-crazy 1970s into early 1980s, gobs of negative feedback were used to broaden/flatten frequency response, lower THD levels and reduce output impedance. These are all nominally laudable goals, but... too much NFB can result a loss of "immediacy" in the sound of an amp, and, poorly implemented NFB can cause "out of band" (ultrasonic) amplifier instability that can destroy such an amplifier when it is connected to a "difficult" (reactive) load. Polk Audio, in the 1970s, for example, sold a very reactive speaker cable ("Cobra Cable") which was a well known amplifier destroyer.
There have also been numerous loudspeakers with impedance curves that make them hard on amplifiers in general, and potentially fatal to some amps (the big Infinity IRS Beta system and the Quad ESL-57 are a couple of well-known examples).
DF of 80 is on the high side of "reasonable", I'd opine... but I use an vacuum tube amp with no added NFB and concomittantly low DF (output impedance ca. 1 ohm).
It's just not something I get too worked up about -- I am way more interested in how components sound than the numbers associated with them nowadays.
YMMV, though, of course.