You don't say how much it costs you; my impression is that the time is more than the cash amount. Have you tried asking your users what they want, or telling your users what you want? I'm thinking of '8500 artists. 1200 upcoming shows. Could someone please buy us a drink?' A subscription model would be ideal but I'm not sure how you do that - although i do think that charging two quid for promotion sends the message that your service lacks value, and that you would have got many more signups if you had demanded twenty. If you can't see a subscription model, cost out an upgrade and some merch and then do a kickstarter to raise the funds.
What sort of values could you ad if you had money to burn? How about a full page ad in NME or Q every month promoting 3 new bands? Or a hookup with a radio station? Have you tried getting sponsorship from music instrument manufacturers/dealers? Musicians are always craving new gear, it's an addiction in the same way that music fans are always craving new tunes.
Great site by the way. It seems like you have a lot of data already up there. Is there a way you can leverage it to make it more valuable? Your target audience is really 2 bases: fans and bands. For the fans: maybe track which band or what venue gets the most clicks? For the bands: maybe provide a resource on how to tour for bands... like a google maps plot of previous tours of other bands... which venues treat bands the best (sort of like yelp)?
General musing out loud:
How about making a "premium" membership for venues? Venues pay a monthly fee ($37/month, whatever) and gain some special advantages. Perhaps they show higher in listings, or get bolded. Or maybe a weekly email of bands which are going to be in the area, as well as any statistics you have about them.
Alternatively, segment out the venues which are updating on a regular basis. Take a portion of that segment and start charging them in some capacity - a flat fee each time they update shows, etc Or make them purchase "credits" in bulk which are then spent on each update.
I think the monthly model is better, since people naturally like the "unlimited updates" that come with a monthly membership, whereas "credits" tend to promote hoarding and may reduce updates.
Another option is customized newsletters via email. I'm thinking a personalized email which shows upcoming live shows in my area. If one of those landed in my inbox weekly I'd probably read it and check out the bands. Once you build a steady open-rate you could start selling that resource to venues (either through advertising or statistics).