To put dynamics and tempo marks on Bach music is fine, but it is breaking tradition. Bach had no concept of the ability of a modern piano, but his music is written for every instrument and thus demands the musican to express it to whatever their instrument is capable of.
If you start adding notes, bring out particular voices over another you will be doing the music disfavour in my opinion. When playing Bach on the piano one should aim to maintain the voice of parts that are not naturally as easy to give voice to, rather than give bais to one over the other. When one start doing that the listener is forced to listen to what the pianist hears, and thus the other underlying melodies are lost or just become a support. This is to my understanding a modernised approach to Bach, where we have an underlying structure which a melody is played on top of. Bachs writing is much more than just that, it can be heard differently every time you listen to it especially his fugues. This attribute of his writing is definatly lost if you start giving bias to this or that.
Czerny marked the 48 Preludes and Fugues with expression marks that Beethoven played with, and one would say that that might have been the best way to express Bachs music on the piano, however the analogy is very much like someone taking a Shakespear play and start modernising it, changing words and acts to their liking (which has been done of course). It is fine but it takes away what traditionally was intended. It may be entertaining and interesting and breath new life into tradition, but it is again on a tangent to what was intially intended. It should be used in small amounts.
This is not to say that Bach has to be bland and played at one volume. There are natural rises and fall in the piece but no one should say that you have to ALWAYS increase here or decrease there. That is not the case, that is why Bachs WTC are so important for pianists, because it forces us to make decisions to the dynamics, tempo and phrasing, because it was never written for piano and cannot be written in one acceptable way, at the same time we have to understand the style of Bach's music and not try to approach it in a modernised way, but try to understand tradition. Glen Gould plays very traditionally, like a harpsichord or a clavichord touch, where Richter plays a lot more modernised in a "pianistic" way.
So anything is acceptable so long you adhere to the context of the peice. If you want to start mucking around with the piece and change things to your own liking you may infuriate judges at competitions. But most definatly you can play Bach two ways, traditionally as the harpsichord and the clavichord, or pianistically with "wider" dynamics and limited piano effects like the pedal. It is about balance, if someone takes a Bach peice and makes it as pianistic as they can possibly do, giving full authority to notes which they think should be brought out, over doing dymanics and tempo changes, you will never win in competition. But in a peformance you may win for sure!
Topic: Bach performance practises (piano)