I've owned the music hall 25.2 DAC for about 1 month and also contacted Gary regarding the mod. He let me know the mod includes a new tube, the JJ6922. He said to try the tube on my own, and if I liked that upgrade, I would like his mod. Well, I just got the European tube installed 2 days ago and it **immediately** made a dramatic difference. Warmer, more weight, more holographic, seemingly deeper bass, though there is a little bit of roundness in the bass as tubes are known to do....that's part of the sound. The JJ is supposedly a new tube that uses the old Tesla factory which is somewhat famous. It has gold pins and it just sounds way better than EH.
BTW, isn't that funny, I also tried the EAT on the Electro-Harmonix tube, and seemed to like it but wasn't too sure. It actually doesn't fit the JJ because the diameter is too small even though the tubes have the same spec.
I'm strongly inclined to get the mod. Gary has good feedback on Audiogon and seems to know what he's talking about. Go to google images and search for Music Hall Dac25.2. You'll come up with sellers that include photos of the 25.2's circuit boards. Compare these pics to photos of modded unit on Gary's site. Indeed, w/o being a techie, it seems he's really adding some premium parts that are immediately apparent due to larger size/different color, especially around the tube area.
Keep me posted if you pull the trigger, and I will try to post my impressions on the mod if I do that in the next week or two. BTW, I would definitely take him up on the A-B comparison/audition since you live in SoCal. Let us know!
Thanks,
Audiohound