A few things to chew on here
It takes approximately 10x wattage to double the volume from a given driver. Currently your running 60w into a single 12 you would need to 600w to double your volume or 6w to half it.
There are different classes of amps you will see class a, b, ab, d ect. This describes the basic circuitry in the amp and how it delivers power it's also why amps with the same wattage rating have different volume levels usually solid state amps arnt as loud as a tube amp.
More speakers more volume. The easiest way to increase volume is add speakers. The average guitar speaker needs about 2 watts for it to start coloring the sound (usually a bad thing but for guitar speakers it's good). So 4x12 cab needs about 8 watts to start to sound good
What I'm getting at is a 15 watt amp into your (I'm assuming) 4*12 should be plenty loud.