For the most part, I agree with Dcstep. That is, once in a certain price range (ie, higher, say over $3K although to some that's not so high), the preamp should be voiced by the designer who knows what he's doing, and should have the guts to say, this is what it's supposed to sound like. A la Shindo above.
Now below some price gear is more, shall we say, flexible, in that budgetary constraints were applied in its design and build, and some amount of tube rolling and/or mods can be beneficial because well, the designer simply could not allocate those funds and still stay priced 'right'. A great example is the EE Minimax pre, which I have owned and enjoyed for some time now, although it does not get much play any more as I do own a nice SS pre over $3K, which is voiced just fine - for now.
Much of what Dcstep has stated above I am totally in agreement with, but am adding the thoughts above. It is IMO quite 'weak' of a designer of high end gear to say, well here is my best shot, but you amateurs can probably make it sound better. Of course no one said that, but some (like myself) interpret it that way when they admit this tweak and that tube will make this $5-10K preamp sound better.