You can't use 18w with a band? In some cases and with some styles, that may well be true, but it can't be stated as a fact. With a 100,000 watt PA and good monitoring, it can definitely work. A couple of things to remember. Unless you're playing those styles of metal where the guitars have all the bottom end, the kick drum sounds like a basketball and the bass sounds like castanets, then the first thing any decent sound engineer will do is put a high pass filter on your channel at around 100 Hz, otherwise the guitar is messing with the kick and bass balance in the system. So you need to enjoy all of that extra thump, because you're the only one who's hearing it. Secondly, the beam effect that comes from a loud amp at distance is predominantly a focused, machete-like laser of 2.5 kHz. Sonically disturbing. Hardly musical. Thirdly, the mic pre at the FOH desk needs to see a few millivolts of signal. It doesn't care how it gets them. Yes, this all changes when there's just a vocal PA, but the most important thing of all is balance. Without reinforcement, you may need higher wattage to achieve it with some styles and some bands, others not so much.
Here's some footage from a gig earlier this year, part of a summer festival tour. 10,000 people, 18w, single 10" combo. Got paid.
Footage courtesy of Brett Kingman, aka Burgerman.
A few seconds of singer preamble before the song kicks in
Cheers............................................ wahwah