It is absolutely very impossible to listen to all the voices of a fugue individually while you are playing it. So to undergo a task to listen to every single voice seperately before you put them together is going to confuse you. What do you listen to? If you start listening to something then it becomes the melody and the rest becomes the support. Now this is fine for other music, but in a fugue it is a lot more than that. It is controlling all these simultaneous voices and finding out how all these voices WORK TOGETHER to create the sound of the fugue. It is a big trap to start thinking of them as individual voices, yes that is fine for orchestra, choirs etc, but piano you can't do that.
A lot of respected teachers of Bach will support this. The preface of the Associated Board of the Royal Schools of Music edition of the 48 bach Preludes and Fugues warn against thinking of Fugues as seperate voices playing over one another.
Part Playing.
The nature of the polyphony has been obscured rather than illuminated by Ouseley's famous definition of counterpoint as "the art of combining melodies." Music "pianistic" fugue-playing has passed as "scholarly" when it even fails to realise that definition, inasmusch as it "brings out the subject" as if all the rest of the fugue were unfit for publication. This notion is peculiar to pianists. Organists, who perhaps play fugues more often than most people, do not find it necessary, whenever the subject enters in the inner parts, to pick it out with the thumb or another manual. They and their listeneres enjoy the polyphony because the inner parts can neither "stick out" nor fail to balance well in the harmony, so long as the notes are played at all. On the pianoforte constant care is needed to prevent failure of tone: and certainly the subject of a fugue should not be liable to such failure. But neither should the counterpoints; indeed, the less often a characteristic countersubject recurs the more important it may be that it should be heard clearly (e.g. the clinching third countersubject of the F minor Fugue in Book 1) Most of Bach's counterpoint actually sounds best when the parts are evenly balanced. It is never a mere combination of melodies, but always a mass of harmony stated in terms of a combination of melodies.
- Principles of interpretation pg12 (J.S BACH Fourty Eight Preludes and Fugues Tovey and Samuel) -