BK~ smooth move on the heater voltage thing :P ...I'm surprised the 6N6P even worked at 12v
MR~ I don't have experience with the hovland supercaps. I've heard good things in general about the hovland musicaps, but not in relation to the CTH. One thing to keep in mind is cap size. You might have difficulties with installing those caps because of their size. Auricaps because of their size fit in easily. Wimas are no challenge either. The Sonicap Gen 2s that I've been using are approx. 1 inch long and just barely fit in a V orientation. The bottom of the V is facing towards the tube socket. You might be able to use a similar orientation and have success.
By the way I just finished another CTH build for a fellow head-fier. Used .15uF Sonicap Gen 2s with success. I'm curious if the .33uF Gen 2s work as well. Sonic Craft is officially out of .22uF Gen 2s. CF3 mentioned to me in a PM that .1uF to .22uF (and maybe .33uF) would work in the C4 position. I'm just curious if there are sonic differences between .1uF and .22uF.
The other thing that was different about this build was that I used 120ohm R18s instead of the BOM 100ohm R18s. The one difference I've noticed is that if I compared the volume dial in relation to "perfect listening level" on this CTH vs. mine I can move the dial up to 2 o'clock vs. 12 o'clock using my AKGK701s.