Pirastro - Flexocore Deluxe Bass String Set, long E (C extension). Flexocor Deluxe is a remarkable recent addition to the Pirastro line. They are fairly high tension, with substantial volume output, and lighting-fast response. Neutral in tone - neither very dark nor excessively bright. Highly recommended for orchestra playing; excellent assist to basses that are inherently low-tension, that are quiet or slow to respond. They seem to be quite durable, and we have seen great results with this string on both student instruments and fine basses.