Edit: after further testing I found out that the Bass Booster shoots two projectiles for every two inspiration, which partly invalidates what's written below, sorry for that! The Midnight Bass Booster does consistently score lower DPS against dummies and monsters than the Megaphone (about 100-150 dps less on average, which I think should be the other way around).
On a side note, the inconsistency between the inspiration costs of the Hallowed Megaphone on the forum and in the game still remains as well.
I'm currently in the middle of a bard playthough and I wanted to make the Midnight Bass Booster, since it seemed like a cool and pretty strong weapon. After actually making it and comparing it to other available bardic weapons, the Bass Booster seems rather underpowered in comparison to the Hallowed Megaphone. The Megaphone is actually many times easier to craft; the Hallowed Megaphone only requires 10 Hallowed bars, and the Midnight Bass Booster requires 10 Hallowed bars, 12 Strange Plating, a Music Box and a Microphone, which is a pretty significant difference. The problem here is that using the Megaphone costs 1 inspiration (this is what it costs in game, even though it costs 4 inspiration according to the wiki) and using the Bass Booster costs 2 inspiration. On top of that, the Bass Booster also deals 2 less damage (which is not much, but it adds up considering the amount of extra shots you can fire with a Hallowed Megaphone). The only compensation for this is the Bass Booster's ability to apply two kinds of buffs.
So, either the Hallowed Megaphone is overpowered with too simple a crafting recipe, or the Midnight Bass Booster is underpowered.
I hope I helped improving the mod a little with this rant right here^