I sing with my playing a lot. In fact, my playing is so poor, my singing actually improves things a good bit.
I have two setups that I use to amplify my voice; in my piano shop I use my Roland KC350 keyboard amp, that has a 3-pin connector for the low impedance microphone chord, and I use a Sure SM58 microphone. Despite not being a dedicated PA system, the Roland KC350 amp with the Shure SM58 mic sounds very good to my old, injured ears.
I also have a Yamaha Stagepass 500 PA system with another the Shure SM58 mic set up in my music room in my house. The Yamaha PA system sounds very good for singing, but not head and shoulders better than the Roland amp. The Yamaha has more settings and adjustments than the Roland amp for a broader range of sound.
I Googled the Roland AC90 amp, and it looks bigger and more powerful than my KC350. Not sure what kind of microphone you have, but I would think the set up would be plenty good enough for your star performers singing!
I'd like to attend your Piano Party event in Bulgaria one of these days, although I've never been out of my home country. I don't even have a passport... but now, they probably wouldn't let me into the country anyway. And if they heard me play the piano, there would be two reasons not to let me into the country