Question about HCA-2 upgrades done by RAM

Hi. Looking at the archives, I have seen several posters recommending Reference Audio Mods for an HCA-2 upgrade, saying it made a huge difference in the overall performance of the amp. I like my HCA-2 (best amp in my system so far) and would love to hear it in an improved state. I found the HCA-2 page of upgrades on RAM's site:

Which of these upgrades result in the largest sonic benefit? I have heard of the $600 upgrade: what about the others (internal speaker output re-wiring, paper-in-oil capacitor option, digital power upgrade)? Which are the most significant for the money, and which are probably not worth the expense on a system that retails at about $10K? Thanks!
Just email Doug at RAM and ask. I think his reference amplifier is a modified HCA-2 .
I have emailed the west coast modder and the general RAM website with questions and cannot get an answer from either one. Is this indicative of their customer service?