Voltage divider getting HOT!
The resistors circled in your schematic are not potential dividers, they are potential dropping or voltage dropping resistors feeding voltage regulators formed by ZD1 and ZD2. If you feed them with 20 V and the Zeners are 15 V devices then the current through them will be given by \$ I = \frac {20 - 15}{220} = 22 \ \text {mA} \$ and the maximum power dissipated in them will be given by \$ P = VI = 5 \times 0.022 = 110 \ \text {mW} \$. 

You have applied 50 V so \$ I = \frac {50 - 15}{220} = 160 \ \text {mA} \$ and the maximum power dissipated in them will be given by \$ P = VI = 35 \times 0.160 = 5.6 \ \text {W} \$. Note the drawing says the resistors are rated for 1 W.


  Therefore I've opened the amp and found that the supply decoupling caps got fired ...


This shouldn't have come as a surprise to you. The engineer that designed the amplifier would have chosen components rated for a 20 V supply, not a 50 V supply.


  What can cause the resistors getting too hot?


We've covered that.


  Any suggestions would be greatly appreciated!


Yes. Gain understanding first. Learn how the circuits work, why the designer chose certain components, do the calculations and then test your modifications.