We ended up picking the unaltered version for most of the things... the two solo ones (just d'amore or harpsichord) had much more empty space and a noisier background... so, just for those two, we opted for the noise reduction.

Yeah... it is really hard to put your finger on it... and it almost sounds right, but just weirdly different. Hollow is the way that I would describe it... but again, you have to be listening really really closely.