Banahan gave Bath the perfect start when he latched onto James's grubber kick for the opening try, although the fly-half missed with the conversion.
The fly-half's break led to the opening try for lock Dan Tuohy and also picked off an Ospreys pass for Simon Danielli to claim the first of his two scores.
But a kick by Humphreys over the Bristol defence was neatly taken by Shane Jennings who went over for the game's opening try under the posts and the fly-half added the extras.
They will be rueing the fact that they played Quins on the day that Evans was making his first start since suffering a knee ligament injury on the opening day of the season - the fly-half producing a man-of-the-match display despite playing in the unaccustomed role of full-back.
Fly-half Jones was the catalyst for Wales' opening points, which he kicked.
Wales fly out to Australia next Monday, and play Canada in their opening Pool D match on 12 October.
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That looked to be it for the opening period with the clock touching 40 minutes, but Blues fly-half Sam Norton-Knight opted to kick upfield rather than send the ball into touch to end the first half.
The visitors dominated the opening stages, with England centre Riki Flutey, playing at fly-half after missing the autumn Tests with a calf injury, Powell and flanker Richard Birkett, back for his first match since October, all prominent.
Rival fly-halves Dan Carter and Stephen Jones kicked two penalties apiece in the opening half.
And their cause was not helped by Matt Giteau's shaky start, the fly-half missing a penalty as well as two penalty touches in the opening quarter.
Montpellier's Trinh-Duc, whose grandfather was Vietnamese, is regarded as a running rather than kicking fly-half and won his first cap as a starting number 10 in the opening Six Nations game against Scotland.
Home fly-half Gareth Owen kicked two first-half penalties to rival Dan Parks' single opening period shot.
The fly-half had a hand in both Ospreys tries, setting up Andy Bishop for the opening touchdown on 22 minutes before finding Alun Wyn Jones on 47 minutes.
Cathay, which last week secured rights to fly the lucrative London-New York route, will also be able to sell tickets throughout China, opening up a new revenue source.
Miller still intends to compete in the season's opening giant slalom in Soelden, Austria, on 23 October, and is now due to fly into Innsbruck on Thursday, several days later than originally planned.
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In the second half Swansea continued to dominate proceedings and scored four more tries, with fly-half Cerith Rees - who ended the match with 15 points - pouncing on a loose ball for the opening try.
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