Found this thread late, so most everything has been covered by Fiddler. He and I have shared notes on mods to the HCA-2, and in fact, he turned me on to Reference Audio Mods which had the best prices for the cpacitors.
Just to emphasize what's already been said, I did the mod in three stages. First, the output wiring was changed from double runs of stock Monster cable to quadruple runs of Jena Labs cryo'd 18 gauge hook-up wire. The improvement was far more than I had any reason to expect it to be. I've posted on this previously, on both AG and AA, so I won't repeat again.
This just whet my appetite for more, and talking to Fiddler I decided to change the four caps in the signal path to the Black Gates he mentioned. After a long break-in period, the amp had been transformed. I now felt it deserved it's Stereophile recommended component status, whereas prior to that, I didn't really think it was in that league.
I probably would have been happy stopping at that level of upgrade, since that represented the biggest, and highest value improvement. But with the amp now so much more revealing, I could hear a few things I wanted to see if I could improve upon. I decided after talking with Kyle at Reference Audio Mods to finish what I'd started, and he suggested Jensen caps to replace the large power supply capacitors. These are supposed to be a better choice for this application than the BG's, and have the added bonus of being less expensive. Since I'd also heard of another guy having a diode bridge rebuilt, I decided to go all the way and have RAM do the same (Harris FREDS). Since these caps/FREDS had to be mounted off the PCB, I had RAM do the work for me (I found they were only about 40 miles away from my location in So. Cal.). I dropped off the amp and the work was completed in a very timely manner (later that week).
The result of the last round of upgrades was more subtle than before, but significant and worth the price. The benefit was the noise floor of the amp had been lowered, and macro dynamics were improved. The HCA-2 also now had a more analog sound, for lack of a better description, and was was really overachieving- to the point that I had to have my Wadia 301 modified (also by RAM) in order to fully enjoy the upgrade to the amp. The Wadia upgrade is another story, but it was perhaps even more incredible. I didn't realize Redbook could sound that good.
I should also mention that I swapped out the Jena Labs wire for the Audio Consulting silver wire that RAM sells. Both sounded great, but the silver is now soldered in place, so I'm selling the Jena labs wire on AG in the next few days. I just want to put the disclaimer out there so that nobody thinks I'm hawking my Jena Lab wire when they see the ad in a couple of days. Both are good, and I'm sure others are too. The main point is this amp has a lot of potential beyond stock.
The bottom line is the basic design of the HCA-2 is great, just remove the most limiting price point components and you can move to a level of performance usually costing a multiple of what you've invested. It's a high-value way to get a high-end sound.
