Doug S. also own these mono blocks.
A few quick notes about the 6C33C. I am building a pair of monoblocks around them, so I've done a bit of digging on them. As Fred said, they are prone to failures, fresh out of the box, as this is semi-typical of gassy low rp tubes. However, this can be circumventing, by doing the proper burn in and prep procedure before plugging them into a circuit, and it what BAT, Tenor, LAMM and other manufacturers likely do to ensure the reliability of the amps they build with this tube.
Basically, you need to wire up the heaters and run them on a regulated heater supply for about 24 hours. The easiest way to do this is to obtain a cheap surplus switching power supply with 6V and around 6A per tube (or 12V @ 3amps/tube, as they can be wired either way). This supply should be current regulated, or at least voltage regulated with the appropriate o/c protection. Most SMPS's have such built in. Just wire up some sockets to the supply on a bench and run the tubes for many hours.
After the heaters have been settled in, it would be a good idea to then bring the tubes up slowly to their operating points, but plugging the tubes into the circuit, but with a variac and brought up slowly (heaters would need to be independent of variac). Then one could even apply a few test signals to run through paces. This last paragraph not all reported to have done, but would probably be a good idea. This ensures any gas is burned off and doesn't cause tube to run away, causing self destruction. Once this is done though, the tubes should no longer have any problems with reliability.