In previous papers, the concept of the modulation vocoder (MODVOC) has been introduced and its general capability to perform a selective transposition on polyphonic music content has been pointed out. This renders applications possible which aim at changing the key mode of pre-recorded PCM music samples. In this paper, two enhancement techniques for selective pitch transposition by the MODVOC are proposed. The performance of the selective transposition application and the merit of these techniques are benchmarked by results obtained from a specially designed listening test methodology which is capable to govern extreme changes in terms of pitch with respect to the original audio stimuli. Results of this subjective perceptual quality assessment are presented for items that have been converted between minor and major key mode by the MODVOC and, additionally, by the first commercially available software which is also capable of handling this task.