This isn't a quit/leave the band kind of issue. He doesn't have 36 watts. He's got 2 EL-84s. 18 watts can get really loud, especially if you have 4 efficient speakers. I've played guitar for 40 years, and the only time I've had to go over 22 watts is when I'm playing baritone. But he hasn't gotten ultra-loud all of a sudden, he's just changed his EQ in a way that the bass doesn't stand out as much. His previous sound was something that a lot of guitar players would have avoided due to its lack of low end, but I liked it, both for personal (obvious) and aesthetic reasons.
I'm still finding out what I like as a bass player, and how it fits in the mix. I like nice tight, non-boomy lows, thick mids, and a top end that captures a lot of click (I play flats), but isn't clangy. I want it to ramp up to a classic 80s punk sound when I dig in, and lean toward old school hard rock when I play normal. It's not rocket science, I just don't understand the available sounds the way I do when I'm playing guitar. I'm really 80-90% of the way there. I'm just looking for some more presence. For now, I'm planning to get an extension cab and mess with pedals some more.