Best portable speaker for an iPhone?

So, my mom's birthday is coming up next month, and she's been bugging me about her volume not being loud enough on her iPhone (Loud, meaning that it doesn't sound like her amazing loud spaced out speakers in her car - go figure, lol). So I want to see if I can buy her a nice bluetooth one that she can use around the house and outside.

What do you recommend? I don't want something with a lot of additional bass and things like that. She's going to mainly use it for 80's, 90's, now alternative, pop, rock music. I also don't want to go over $150 (less than $100 even better because she'll probably blow through it quickly), and I'm open with buying used/discounted on eBay.

Thanks.