Hello, everybody. I'm a long-time reader and new member. I have both a Signature 30 LFP-V and an Isabellina LFP-V, playing through Omega Super 5 XRS speakers, with an Omega subwoofer. I got one of the earlier LFP-V Signatures, I think, and added the Isabellina some time later. The Isabellina was a massive improvement over my previous DAC, an Arcam rDAC with a custom power supply - much fuller, more natural, and less/not at all "digital" sounding.
At Vinnie's suggestion, I've replaced, at separate times, the stock tube in each unit with an Amperex 7308 vintage gold pin. Each change was completely for the better: more natural sounding, more detail, better separation, better extension on both ends, better soundstaging and imaging, more air. I haven't seen any drawbacks at all.
The change in the amp took a while to really show itself - the new tube sounded better right away, but not a lot, and then came in over time. The change in the Isabellina, though, was instant and massive: I was really shocked. It continued to improve over time, too.
I can't compare any of this to other tube upgrades, but I can say that these are terrific.