ARC EC21 tube Crossover SN# 71112008 in excellent condition. Original German 12AX7 tubes. Crossover set from factory at 400HZ according to the back panel but it checked at 2000HZ. No documentation. You can get it from ARC for under $15.00 to set your own frequency and slope. Why pay for over a thousand for a Bryston 10B when you can get this for half the price! Check my other ads of ARC SP6B and ST70-C3 from the same owner. Paypal add 3% plus shipping. US sale only.