are they 2x2 or 2x4 tiles? 2x4 sag more quickly, mostly due to the span. if they are 2x4's, you may be able to add an intermediate cross bar to break the spans to a 2x2 grid
sounds like (in addition to maybe a dehumidifier?) you need a sag/humidity resistant tile; something from one of these listed in this link perhaps (hope this link works
https://www.armstrongceilings.com/c...ble-ceilings/humidity-resistant-ceilings.html
- Ceramaguard is what you'd see in commercial kitchens, which tend to be SUPER humid/hot/wet...they're heavy MF's (don't think you need these)
- Cirrus is good choice. we'd typically use those in classroom/office/boardroom scenarios. they have the anti-sag/humidity resistance, also have a good sound absorption/sound blocking rating. and they've got a good reflectivity rating. they come in different colors, so if you wanted to dress up your space or funk it out a bit, you could mix white/black/brown etc.
anti-sag tiles = they are expensive compared to non-sag (which is likely what you have now).
one thing to note...whatever acoustical ceiling tile you put in place will likely rain dust when you lay into your marshall, save the ceramaguard, but hopefully not as badly as what you are experiencing now (old tiles, which are very brittle)
as long as your grid isn't swinging or swaying or anything, a simple tile swap will work for you.