So, I received my TRP upgrade two weeks ago and have done a ton of listening since then. For tubes, I have been using Gold Lion KT66s. Thus far, those are the only tubes I have tried so I obviously can't provide any insights into the relative merits of different tubes. I do, however, look forward to eventually playing around with different tubes. The ability to tube roll is definitely one of the great things about this new dac.
I am extremely happy with this upgrade. My original Atlantic (balanced plus, dsd only, 512 capable) was outstanding. Nonetheless, the TRP version is a serious improvement. In comparison, the TRP has a far more natural sound with more realistic and engaging tone and timbre. Even though the original did not sound "digital" as such, the TRP has a far more analog sound. The music flows with the ease and immediacy of a good vinyl system but with the superior detail and dynamics of digital. The TRP also seems to have a richer more tonally dense sound. Interestingly, virtually every other new component I have previously added to my system has produced some tangible improvement like better soundstage and imaging or more detail. The TRP's improvements are less tangible, but are in many respects more impactful. It has added a level of refinement that I have not heard in many other systems. A great product.
The What's Best thread that Gopher identified in his post on this dac is really interesting and helpful for those interested in this upgrade. Tons of information on tube rolling.options.