Except for a startup crackling in the Left channel which I've tracked down to a worn out socket on the X-101-B( checked & tested with 4 different sets of tubes including 2 T-S's, 1 set of EH7591's, and a quad of Westie's) I love them. I've got quad in the Executive Model 950 (1963), a Sansui 1000A, in addition to the X-101-B. They had the most impact on cooling factors in the Sansui where the construction of the chassis basically encases the tubes on 3 sides with transformers, block off plates, Power resistors(which were moved after the 1st quad), and Tuner Front end. Lousy layout for tube cooling. I've managed to get temps down into the low 130's now instead of the original 160-170 region on the Sansui. Due to the smaller envelope they leave more room between tubes in the FISHER's which help with cooling on the 800c's. They DO tend to run a bit hotter for a set bias setting on fixed bias. Cathode bias I haven't checked yet although I did set mine to 43V and they've been running for over a year and without problems. If you do get a quad, I highly recommend Jim McShane as he tests them to higher levels than the factory or anyone else I've seen.