A friend has the Tron 7. I've heard it in his system and in another friend's system. It is a very nice phonostage.
The manufacturer insists that it has been tuned specifically for the tubes that come with the unit and that alternatives will not sound better. I think that what sounds "better" is a matter of taste/system matching and it is actually unlikely that what Tron supplied will be "optimal."
In both friend's system, all three listeners, myself included, preferred different 12AX7 tubes than what came with the Tron. The owner ended up going with Amperex Bugle Boy 12AX7s (the other friend has a VAST selection of alternatives). I would suggest at least trying some alternatives.
We did not mess around with loading. The sound happened to be decent at the manufacturer's preset loading (I believe the cartridges in the two systems were a Lyra Helikon and an Ortofon Per Winfield.
I too wonder about manufaturer's that don't make it easy to try different loading. From experience, most cartridges sound acceptable with loading of 100-150, so one can get away with fixed loading somewhere in that range. Still, with some setups, I actually like very little loading for a more wide open sound (e.g., 47k loading).
You could at least experiment with loading by either soldering in additional resistors yourself. If the Tron 7 has a step up transformer at the input (I believe it does), you will have to decide where to experiment with loading. My own phonostage (Viva fono), has a step up transformer and fixed loading. I found (by opening it up) that the manufacturer had placed loading resistors across the back of the input jack. In other words, the loading was on the primary side. The alternative would be resistors after the transformer (secondary loading). I disconnected one leg of the resistor so I could experiment with differnt loading. I then bought Radio Shack RCA jacks and adaptors, and a variety pack of resistors to make plug in loading jacks with different values. Interestingly, I ended up going with the value that came with from the manufacturer (actually not that surprising since, I found out later, the unit was optimized for the Transfiguration Orpheus, the same cartridge I use). With step up transformers, you can drive yourself crazy with experimenting with loading because, as mentioned above, you can try loading before or after the transformer, or a combination of both.
If you are not that technically inclined, don't mess with loading with a unit with fixed loading. If you have the itch to experiment, start with changing the tubes.
