Hello tubestuf. Thank you for your comments. First let me say I am not trying to reinvent the wheel here. I also defer to Jim on this, however what I am trying to accomplish is getting the best sound, (to my ears) out of my system. I listened today for an hour with the 5U4. I kept checking the transformer, and it did not seem to be getting too hot. I will machine my C2's transformer cover to hopefully vent away excess heat. I was also a bit off in my initial statement regarding volume pot position. It's around 11 o'clock vs 1 o'clock for the 5Y3 tube at the volume I normally listen too. This seems to be in the "sweet spot" for my tube amp. I have also read the posts relating to other rectifier tubes that work in the C2. Unfortunately I do not have any of these on hand. I stand by my previous post that the 5U4 tube sounds better. More bass. Cleaner fuller sound, and less hiss and hum. Most likely due to my amp not being close to its clipping point.
Today I received the Granite Audio Phono Stage burn in CD. My C2 is cooking as I write this, (with the 5Y3 installed). I have also reinstalled the removable ground pin on the power cable feeding my C2. After rerouting and twisting wires the hum is gone...for the most part. I agree that there may be a ground loop elsewhere in my system as Jim has suggested. There is definitely some transformer hum from another unit. I have ordered a R-Core transformer to correct this. Let me also say the "hum" I am trying to remove is very, very small. It is just higher than I am used to hearing in my system.
I have also ordered the 370EX transformer, and will post my findings here. For me, the great thing about this hobby is that there are no absolutes! What sounds good to you may not sound good to me. I believe that is why we are all here. To build and modify our equipment to be the best they can be, and this forum allows everyone to exchange ideas. That said, every component in my system is highly modified. From my Altec Lansing 604E's to my Emotiva ERS-1CD player, to my point to point tube buffer stage and Xindak tube amp. I also have two highly modified phono stages. A Jolida JD9 with two separate power supplies and a 36db/Octive Subsonic Filter and a Pro-ject PhonoII that is also modded.
If you have looked at my other post you will see that I spared no expense (within my budget) building my C2. A rough total is $1527.19 + the power cord and the new transformer. I tried building it for "around" $600, but I exceeded that on my first parts order. I will also be purchasing a really good fuse. I have Hi-Fi Tuning fuses in the rest of my system and they "sound great". I will be installing it after all the mods are done.
I also feel that things posted here should be on a positive note. I always try to be respectful and polite on these forums. I also do not offer an opinion on something I have not personally tried. There is something to be said regarding thinking outside the box. I hope that is what I am doing here.