But they took the reigning champions Toulon close in December, losing 20-16, and have since gone on to win their last three GUINNESS PRO12 matches.
Today's visitors to the RDS, Bath, are currently five points below pool leaders Wasps with two group games remaining.
And Cullen told the42.ie: "Bath are reliant a little bit on other results but they'll be looking at getting two wins, ten points.
"I'm assuming they'll come here with the ambition to play - I think it'll be another close game.
"Plenty has happened since then (the defeat to Toulon). We're hoping for a big improvement.
"We were gutted after the Toulon games, I look back on them all the time.
"There are parts of the games where we just mismanaged certain things, or if certain things go our way it could have been very different.
"It was always going to be an incredibly tight group and unfortunately it highlights just how fine the margins are at the top level."
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