So I started a band recently, but the bass player still only has a 15 watt bass amp. He is new to the bass, and I don't know much about it, so my question is: How loud an amp does he need to be able to play over the drums, guitar, and horns? About how many watts does he need (I know that watts does not measure volume)? Can you suggest any amps that are good?

I think the minimum is 100w. Depending on the guitar players amps tho, you might need 200w or more possibly.
30W tube is going to need lots of bass. i have a LOUD drummer and im running 300W. upping to 450 ideally
Get off this damn forum and play your damn guitar.
100w 1x15 or 2x10 is usually where everyone "starts out" but if he progresses fast or begins to gig regularly he will probably want something above 300w....there are some very good combos out there boasting 200+ watts and multi configs of speakers for not much more than the "standard" 100 watters. Fender Bassman series, Acoustic Brand (new inexpensive decent loud), Ampeg BA Series (not alot of people like em i owned a 2x10 220w and i loved it), STAY AWAY from cheap Gallien-Krugar stuff (it all looks great and the price is low but so is quality,volume, and tone) ALSO STEER CLEAR of Behringer its practically a bad word amongst bassists...

Thanks for the help.

Anyone else have any advice?

Also, the guitar amp is 30 watts all tube.