For nearly 10 years I have use a single Bose L1 Compact speaker for all my gigs. I too have jobs where 10 -20 might show up and the rooms are small. I also, like today, play large venues where 200+ people are present. I find the Bose does wonderful for both gigs. All I do is use room appropriate volume settings and that works just fine. I seldom get complaints about the sound being to loud or to harsh. I do not use a mixer and use the onboard EQ system of each keyboard to balance the sound. I have used this setup with Yamaha PSR3000, Tyros 3, Tyros 4, Korgs PA900, PA1X, PA2X Pro, PA4X Rolands E80, E50, and now my Ketron SD7. It worked just great with all the brands and models.
What I have found is the volume level is the most important aspect for the senior audiences.

Hope this helps,
Deane