The authentic instruments movement, which has of course been around a long time now, poses some interesting philosophical issues.
Both materialism and Hegel (as with Spinoza) are 'monist' systems. That is there is only one (ontological) level of reality reality and divisions into dualist (mind and spirit - Descartes and 'everyday common sense;, phenomena and noumena - Kant) is rejected. This means there is no 'essence' existing apart from that which actually exists.
This means from an aesthetic point of view concepts of 'the spirt of something' versus 'the letter; of something, or 'the spirit versus the notes' in the case of music, cannot be defend. There is nothing which exists apart from/over and above that which actually exists.
This means a number of interpreters have concepts which correspond to a Hegelian point of view. Celibidache's insistence that the reverberation and accoustic properties of a venue were a significant factor in interpretation was entirely correct from a Hegelian point of view - in altered reality and therefore that which existed.
More generally the authentic instruments movement with the diference in balance of instruments etc achieved the same result. The difference balance in Mozart, or the tempo of Beethoven, significantly altered reality.
But there is, however, another issue. Reality consists of a totality - that is causation within it is determined by all elements. Furthermore not all elements are of equal strength - that is the totality is structured. Merely to use authentic instruments, not taking into account the weight of the elements they bring into play, is therefore not sufficient to ensure the totality is 'authentic' - that is corresponds as closely as possible to the reality created by the composer.
Further issues are therefore posed here which will be looked at in subsequent posts.