That is a good question! It will certainly be quiet and with the 6dB-12dB gain and VC, it could replace a preamp!
It won't have all of the features of a preamp, including R/C I would say, although that could be done as well. It is just that pretty soon, what started out life as a simple box is now as complex as a preamp
. Also the cost increases.
That is why the direction of this conversation is good and is helping me to decide and shape the concept of what this product will be! At first, it was simply intended to be an 'outboard' tube mod for use with any number of different digital products, rather than being fixed to one. Of course it is obvious that it not just be limited to digital sources and with a few simple features, could also replace a preamp.
To keep the design simple, the following are key I believe:
1-2, maybe 3 inputs.
2 outputs, optionally XLR fully balanced outputs ($$)
Transformer coupled gain, bypassable and configured for 0, +6dB, +12dB.
Tube buffer stage for Zout < 100 ohm.
Optional volume control.
I am thinking of a one-box solution, again to keep cost down and keep it simple.
To answer your question, I believe it would be as quiet, transparent and musical as a good preamp, yes.