There's a really hard problem when you're dealing with an online audience of classical musicians + fans/supporters in a social media setting. The problem is that the people who are most knowledgeable and passionate about classical music are mostly the performers, and classical musicians are a terrible demographic for making money. The basic problem is that your most enthusiastic users have no disposable income. The vast majority of classical musicians live a gig-hustle lifestyle + private teaching, and supply
far outweighs demand for classical performers and teachers. I did that life for 20+ years and was pretty good at it, but there's just not a lot left over at the end of the day. Full-time salaried positions for classical musicians boil down to low-paying university positions and decently paying professional orchestra positions, and both are extremely hard to get and very limited in quantity. So if you're buyer has any sense, it's going to be hard for them to figure out a business model.
That means if you want to sell, you have to pitch the business model. Aside from the actual musicians, you probably have as some part of your user base, classical music supporters. And these people are where the potential money is made. This are two segments. One is the traditional arts supporter: Older (60-65+), white, Democrat, female, tons of disposable income. Your user base is unfortunately not likely to have much of this segment. The next segment you want to look for among your users are people who used to be musicians that have moved on to other, more stable and lucrative careers. That group is disproportionately male, in their mid-30s to mid-40s, left-leaning politically, has disposable income, and is generally pretty eager to spend because they used to be really broke and are typically interested in treating themselves to nice things.
That's going to be the bulk of your value proposition if the business model is going to be to monetize your users directly. So I'd try to figure out approximately the size of that demo in your user base. When I worked in the market research industry, I did a fair amount of pro bono work for several orchestras around the U.S. studying their audiences and trying to help them figure out pricing models. That was about 10 years ago, so my data is old. But given the nature of the beast, I doubt they've changed substantially since then.
All in all, I think you have a tough proposition for a potential buyer in the sort of traditional, "I have the eyeballs, now show me the money." kind of model. But I would encourage you to maybe consider some other options.
As I said before, there is an entire class of people--basically the older, white, liberal aristocracy--who support classical music and the arts almost out of a sense of class obligation. You (or a potential buyer who was motivated) could try to find an investor willing to fund this project at a very moderate rate of return as a sort-of "break-even + a little extra" investment that also promotes the arts. Your business model there would eschew advertising because it's not really going to give you much of anything as income anyway and just annoy your users. Instead, you'd need to find some value in premium features such that you could charge a subscription for them.
Along similar lines, this could be spun off into a non-profit with a mission to spread the beauty and joy of classical music through social technology. You (or again, buyer) might have a lot more success here, especially if there are existing connections with the donor community for classical music.
The last idea before I switch gears is that you might be able to sell directly to an Orchestra. It's true every orchestra around the world is struggling financially. But these kinds of projects are things that donors can get really excited about. Arts organizations everywhere are trying all kinds of different ways to improve audience engagement and mostly failing miserably for reasons I won't go into here. But I could easily see the board of a major orchestra diving in and using your existing audience and technology to try and juice their attendance and revenue through various partnerships + really targeted ads.
Switching gears for a moment. . . . This is going to sound totally nuts, and I don't even know how to start down this path. But it strikes me (as well as everyone else, probably) that Apple is absolute shit at social networks. Especially social networks revolving around music. You could make a strong case for being acquired by Apple on the grounds that you've built up a sizable audience in a very difficult market without blasting ads or invading user privacy--all values which align with Apple in general and where possible. How to go about getting to have that conversation, I'm really no help at all. But maybe someone else here does.
I really wish you the best of luck. I love this project, and if I had the cash I'd buy it off you, quit my day job, and figure out how to make this work right now. Feel free to reach out by email in my profile if you want to talk any more about this or anything or if there's other ways I might be able to help.