If you control the client and are able to update it, I think you're better off switching to opus or similar.
96kbps opus is actually quite good for music already: https://wiki.hydrogenaud.io/index.php?title=Opus#Music_encod...
I guess the question then is whether there's a market for 48kbps opus music. But that'd be assuming that a big part of the costs are bandwidth. I'd bet that's not the case.
Also imagine how little traction you'd get once people associate your brand with poor music quality. I don't think you can compete by being 2€/month cheaper.
I can't see any reason for 128kbs service unless you're absolutely starved for data. There is no other advantage.
A lower price tier? Or even a Freemium model. Considering most of them are already available on Youtube. As strange as this may sound I dont think Streaming is the business model that will save Music industry. And despite the headlines ( whatever MSM likes to print these days ) most artist's real income are from somewhere else.
You're only making that problem even worse by offering a low quality, even lower price model.
Frankly I can't imagine there would be many artists particularly happy with having huge swathes of people introduced to their music at 128kbs.
If you could, then yes there is a quality differential, rather you got it right or wrong may be subject to taste. But at least we established there are differences.
But when we say 128Kbps are sort of CD Quality or CD being no-difference to 320Kbps mp3, they generally refers to vast majority of consumers, not audio experts. I think most consumers could tell the difference between 256kbps AAC and 128Kbps MP3. But beyond that it sort of goes into consumer dont give a damn territory. Although most Pros would want the highest quality possible and there are no reason why we cant deliver lossless with today's infrastructure.
I am wondering if there is a market for an even cheaper Music Streaming Services that target lower quality, 128Kbps music.