Dude the chasis is aluminium which is a better heat dessipation than an heatsink. Try an tda2051. But it may need an upgrade of Transformer. I guess your jarring depends on a loads of things. I have had a LANEY LX12 and Marshall MG10. which is even worst than Kustom 20. Its very easy to jar stuff. If your input gain is higher it shallj ar even an 50 w amp. Guess you need a bigger transformer. Try to put in an torriodal transformer with atleast 1.5times the current rating.
do photos count as "reality"? here's the check: forum ethics: search first. think. think again. then type. this one is louder than a kustom 20, looks a bit better IMO and cost me a lot less than 2k to build (or would have even if i didn't have the parts lying around): i won't honour the kustom 20 by comparing the sound of the two. to see what it sounds like, first and last clips on this page: https://www.soundclick.com/bands/default.cfm?bandID=1046495&content=music now reality check THAT!