That's good news about the hum. I have never seen a pair of RCA tubes with black glass before. I'm thinking those are not RCA made - probably National Union? or maybe Ken-Rad? In any case, RCA date codes can be hard to figure, but I am guessing that yours are made in the 17th week of 1956, based on this from the RCA Date Code guide I have:
month indicator comprising the RMNEIh week code, quantized to 04 (Jan.), 08
(Feb.), 52 (Dec.), etc. Included is a "sales basis" code: a dash between year and month
indicated tubes sold on an adjustment basis; no dash indicates sale on an allowance-inlieu-
of-adjustment basis (i. e., renewal vs. OEM business). This scheme covers privatebrand
tubes, except where a buyer's special code applies, but not Government tubes.