Complex sentence: Townshend had been using Sound City amplifiers since mid 1967, and Entwistle since earlier that year, when in late 1968 they approached the company asking if their equipment could be modified slightly.
Split into simpler sentences: Townshend had been using Sound City amplifiers since mid 1967, and Entwistle since earlier that year. In late 1968, they approached the company asking if their equipment could be modified slightly.