Here's my "dos centavos": Let them play the gig. It sounds like you guys are a bit younger than this other band, and hopefully the sub will have some gigging words of wisdom for your buddies. This means your band will be better in the long run. Remember also that your local music scene is all about networking. Just because the other band has a monthly gig at this club does not mean that they haven't played other places or don't know other bands. That can be invaluable. I think most of the folks here can agree that a good band to band rep can land you a few gigs along the way. For example, in my early 20's the guitarist from one of the bands we played with broke a string and I told him to use my guitar to finish the song (I was standing next to the stage so I pointed to mine). He managed to finish that song and they played the following with my guitar. While they played that second song I restrung his guitar and they finished their set. I didn't think anything about it until about a month later when guitar guy got a hold of me through the club owner, and they had recommended us at another venue where we walked away with around $100 a piece. So that's networking paying off. Also keep in mind if you develop a good relationship with this other bass player you may one day get the call to be HIS sub later on. Not a bad deal. Having a sub is kinda scary because it feels like you're easy to be replaced, but it also means that you have the option of taking a break if you're feeling sick or burnt out and just need a week off, etc... Let them do the show and enjoy your vacation.