The unit that made Quad: a 15 W mono block with an ultra-linear output stage, sold as a pair for stereo and still the reference point for British valve sound.
UK valve chassis: mains-derived HT rails stay charged after switch-off; original Hunts/TCC/Erie capacitors and the mains transformer are the two failure points. Value is set by untouched transformers and original cosmetics (silver/black livery), not by the valves fitted.
Marks that can be entered for this maker: Serial number, Model / catalogue number, British capacitor code (Hunts, Mullard), Japanese capacitor code (Nichicon, Chemi-Con, ELNA), EIA component code, Version & era marker.
Nothing predates 1953; components only push the date later.
Sold-price aggregates are not collected yet, so the archive gives no figure — condition, originality of the transformers and a documented recap move this model more than any list price.
Evidence: Dada Electronics — Quad production history & serial numbers (PDF) (PDF) · Keith Snook — Quad II restoration research · +1 more