I've been reading up on the fix, but at least so far I haven't found exact details for applying it to PF20, or if anyone has done it... In the original humbuster thread someone just suggested that the PF20 would need different component values. Maybe an amp tech could work it out from the basic idea.
I paid well under the usual retail, which I guess makes me less inclined to think about returning the amp. Which does feel a bit daft.