I also have Cary 805B monoblocks, no you can't use the 845 instead of the 211 without a modifications. Cary will upgrade to a 805C for 2500.00 or they can just make a change with a switch which allows you to use 845 output tubes without the full mod. to 805C. I replaced the stock 211 tubes with NOS RCA 211s and it was a major improvement. I had a chance to take home a pair of 805C for the weekend for a trial. While they sounded different than the 805B, I can't really say they sounded better in my system. I decided to stay with the 805B. You can contact Kirk at Cary or feel free to e-mail me with any questions.Good luck
