Here are some other artists to whom Howling Bells have been likened:
Yeah Yeah Yeahs (female singer)
The Duke Spirit (female singer)
Blondie (!) (female singer)
Nick Cave and the Bad Seeds (Australian)
Garbage (female singer)
Apart from the obvious comparisons I have put in brackets I can't really see any of those. See this thread:
howlingbells.proboards.com/index.cgi?board=howlingbellsgeneral&action=display&thread=150started last November where we asked this question and I think we eventually came up with the answer that nobody else sounds like Howling Bells, 1st or 2nd albums.
I only found Mazzy Star after I discovered Howling Bells were influenced by them (and I now have two of their albums) but apart from the female singer thing, they aren't really alike.
Trespassers William? Never heard of them. Must have a listen.
[25 minutes later edit] Listening to Trespassers William now. They're very good, but similar to Howling Bells? Not really. They're more like Ether Aura (see the above thread), and I can see some Mazzy Star in there.
And, by the way, that thread introduced me to Halogen, who are now in my top five artists ever. And Trespassers William played in Duisburg on October 15th - that can't be too far from you. The main similarity to Howling Bells is that they're signed to Nettwerk.