how to convert this amplifier to an 15/18V supply?

Your amp will dissipate the full power all the time, regardless of the output . It's class A .
here is the heatsink, im gonna use one per transistor and i wont bother with myca sheets becuse well its one per transistor.
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At the input of the circuit, if you use a 10k pot.
But a low impedance buffer stage after the pot will be a good idea.

The bias adjustment may cause an imbalance between channels in the pot
without a low impedance source.
 
from this first prototype i know that heat is an isse and the biasing can get very off becuse of it id also need an bigger source resistor for the current source to run it at even one volt more, it works but id need to invest in a bigger heatsink, significantly bigger.
 
These heatsinks will proably sink max. 3 watt heat)-;

For the desired power (15V, 1 A ?) I recommend TO-220 transistors. They run without any problems at this power and sound much better (higher resolution, better interplay of frequencies as sound, cleaner...) than the TO-247 MosFets. MosFets should not be allowed to run too hot in the long term, as they become sonically rotten. I currently have an SE: TO-220 at 34V, 2 A running, but BJTs, to my ears they also sound much more elaborated, more physical, more organized.... than comparable MosFets.
 
These heatsinks will proably sink max. 3 watt heat)-;

For the desired power (15V, 1 A ?) I recommend TO-220 transistors. They run without any problems at this power and sound much better (higher resolution, better interplay of frequencies as sound, cleaner...) than the TO-247 MosFets. MosFets should not be allowed to run too hot in the long term, as they become sonically rotten. I currently have an SE: TO-220 at 34V, 2 A running, but BJTs, to my ears they also sound much more elaborated, more physical, more organized.... than comparable MosFets.
if i were to use bjt's id need a gain stage such as this one or an single transistor and an transformer
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should i put an capacitor between the buffer and the input?

Yes, there already is an input capacitor in the diagram, since there is a DC voltage present at the input.
If the bias pots were at about the same setting, you could use a 10k stereo linear pot
and let the input impedance convert it to quasi-log. But if the pots are set very differently,
there will be a channel imbalance due to that difference, so a buffer may be needed with an input pot.