The Reds had lost scrum-half Jamie Paterson from the previous weekend's side, while Alex McCutcheon and Stuart Miller were also out, meaning a backline reshuffle.
2nd XV captain Alex Watt was promoted to the left wing, with Ewan Chalmers recalled on the right, as Kyle Adams returned at full-back.
James Ferguson moved to stand-off to accommodate for Nick Bell's selection at scrum-half.
In what was effectively a 'dead-rubber' for the hosts, with nothing but pride to play for, they got off to the worst possible start as stand-of Mike Ker added an early penalty to make it 3-0.
From the restart, full-back Ally Leddingham carved open the Reds' defence to score his first of five tries.
Wingers Chris Berkery and Stuart Niven helped themselves to a pair each, benefiting from some superb counter-attacking, led by Leddingham.
Their other scores came from scrum-half Matt Page, and replacements Gavin Brown and Tom Hart.
Stand-off Mike Ker added 24 points of his own with a try, penalty and eight conversions to seal their place in the cup semi-finals.
It was 36-0 at the break and, despite the hosts' best efforts, the scores kept coming in the second half, as Marcus Di Rollo's side ran riot at Neilson Park.
The hosts rallied late on and managed to grab two consolation scores.
Ever-present lock Ben Laing crashed over for his first try in senior rugby, and a first for at least three years, before Chalmers rounded off a well-crafted team move.
Nick Bell, returned from the sin-bin to kick one conversion, but they were merely consolation scores as the visitors returned to Myreside with an emphatic victory.
After the match, head coach Bryan Craig said: "We were outclassed by them all over the pitch today.
"They are a very good side and we gave them far too much ball to counter-attack with, and when they did, they scored."