Of course, if you were to get one of those amps with bluetooth, the cheapest and easiest way to play your music would be if the phone you already own has bluetooth, with enough storage to hold the music you want. In my case, I have an older i-phone with only 16 gigs storage, but it's still enough to hold my favorite music. But it is approaching full capacity, so if I were getting a new phone today, I'd get at least 32 gigs or more, if the price wasn't much higher for more storage.
Or you could get an Ipod or other MP3 player with bluetooth, and use that as your dedicated music storage. But, I think having it on the phone is more convenient, so you only have to carry one device.
I buy my digital music from Amazon, and with an Amazon app on my phone, the music goes to the phone. I have Amazon Prime, and they give you a lot of the old songs for free when you're a member. The ones I pay for are usually a dollar each.
I have some CD's, and I used to copy them to the computer. And from there I could add them to my Amazon app. But Amazon discontinued that feature, so while I was able to keep the ones I already downloaded to the app, they don't let you do that anymore. But, any new songs I want, I just buy them from Amazon, since they don't cost much anyway and it's easier than it was to transfer music to the computer and then to the app.