How can you tell a classic product from the hi-fi hoi polloi? One sure sign is when third-party developers spring up around the thing, offering parts and service intended to maximize its performanceor just to keep it on the road. Thus regarded, a few true classics emerge: Quad's ESL and ESL-63 loudspeakers. Altec's 802 and 806 compression drivers. The Linn LP12 and Garrard 301/401 turntables. The Rega RB-300 tonearm and its direct descendants.