Ok, so I just received my new 6J1P-EV tubes yesterday. Installed them into my Little Dot MKIII headphone amp. With the DT880 600 ohm headphones, there is a slight increase in bass over my Mullard driver tubes. And I mean slight. I'm a little confused when certain people describe changing driver tubes as resulting in drastic sonic changes. This is my 3rd variety of tubes that I've tried with the headphone amp, I supposedly have one of the more revealing headphones out there, and I hear all of these tube changes as slightly altering the sound characteristics. There may be a small percentage of the audiophile community that can easily pick out these changes, and then they may describe these changes as "much increased bass", or "sound stage increased by 20 feet in width",etc. Compared to all my friends and family, I have the most discerning ear (that may or not mean anything), and have typically been highly critical when things are not balanced well. I still don't hear these large changes in sound characteristics that supposedly should be happening. Who knows, maybe my ears are somewhat flawed. According to my latest hearing test, all is good.
The odd thing is that my DT880 600 ohm phones have shown me how much I love my lowly SR60 cans. With the new tubes in my LDMKIII, the Grados have noticeably more slam and bass than they did with the Mullard tubes. Odd that I noticed the difference in tubes more with my cheap Grados than the DT880's. Even more odd is that people told me that I would not notice the changes in tubes with my lowly SR60's. I know what many of you are thinking...the DT880's are not a rock or techno headphone and I should not expect them to be bass heavy. I don't want them to be bass heavy. So far my experience with the DT880's has been mixed. I would describe them as having a very airy and delicate sound. Highly detailed. To my ears, the top end is too hot. Everything has too sharp of an edge to it. To my ears, even live music doesn't sound this sharp-edged. They also seem to lack any form of warmth. The DT880's really help me to analyze music, but I don't know if it helps me to enjoy the music and get into it. To me, music always needs to be heard and felt (a little). Maybe I'm finding I don't want music to sound airy and delicate with copious amounts of detail. I'm not going to give up yet. Perhaps I will adjust to this and end up loving it. That or maybe I just need to get a good EQ. I hope this doesn't come across as too negative of a post. Just thought I'd add my experience.
Sounds like you are ready for the close-back mod. Don't worry though, that's a good thing! :-D