Further updated advice from Hacker (in this thread
http://www.pinkfishmedia.net/forum/showthread.php?t=151178):
So yes, the single best thing you can do at present is ditch the film feedback cap (heresy!!) for a wet tant.
Feedback cap (C2)
Tantalum, axial, wet, at least 68uF. Voltage not important, there's only a couple of mV across it. Do this if you do anything, it's the best upgrade I've made to my amps. In fact I'd go as far as saying: do not put film feedback caps into a HackerNAP. Period!
C13 and C14 - Output transistor decoupling caps
If you can find them and make them fit (definitely not a drop-in mod!), remove the 470uF output decoupling caps (C13/C14) and replace them with 22uF - 68uF polyester film caps rated for 63V+. The difference this makes to bass clarity is very worthwhile. I used obsolete/unobtanium 47uF/63V Evox Rifa MMK for this.
John Luckins' VBE mod
I'm too lazy to link to it, but this is a great mod that vastly improves (reduces) the output impedance of the front-end VBE filter, which in turn improves bass response.
HackerCAP - Output stage
Ditch the inductors and use chunky wire links instead. This keeps the output imepedance as low as possible. Only do this to the output stage HackerCAPs, not the front-end ones; those should remain heavily filtered with CRCRC.
C25 - CCS filtering cap
There's approximately 25V across this cap, which opens up the possibilities for higher capacitance. I moved to a 10uF/50V Wima MKS2, which just drops in.
Getting silly - ****y caps
The front-end power rails benefit from the very best decoupling caps you can find/afford. Currently I use brass cased 1uF polystyrene NOS Russian caps at C19 and C11. These are rarer than rocking horse shit but a bargain if you can find them on eBay, etc.
Also worth doing is the input cap. I use the obsolete/unavailble Evox Rifa SMR polyphenylsulphide caps. You could usefully use a polypropylene type here, although good luck getting it to fit on the PCB.
This list is based on order of preference, first to last. Every suggestion except the ****y caps is something I would consider a "must do" mod. The fancy caps are really just like the salt and pepper - flavour according to taste. The rest of the changes make a fundamental difference that shouldn't be missed.
There are more changes in the works. For example, John Luckins has been working on a hot-rodded SuperReg arrangement for the front-end supplies. I fully expect this to be another "must do" mod.
http://www.pinkfishmedia.net/forum/showpost.php?p=2227677&postcount=20