Illumina 1.8 corresponds to Sanger encoding
Starting in Illumina 1.8, the quality scores have basically returned to the use of the Sanger format (Phred+33)
The Stampy manual states that:
By default Stampy assumes that the quality scores use Sanger encoding (score 0 is '!', ascii 33), and use the phred (=log p) scale, not the logit (=log p/(1-p)) scale.