When you look at the tubes, you say one looks slightly darker than the other. Are you describing the condition of the tube when off, or the filament brightness when on?
Without other information, I'd say you have one tube that probably had a manufacturing defect. Welcome to the wonderful world of tubes. I bought 4 NOS Chatham 2399 tubes for my WA3. Two sounded pretty good, with a little residual noise, one sounded very good, and one had a manufacturing defect that caused it to arc over, causing the connected headphones to make a VERY LOUD POP as if someone had fired a .22 caliber revolver. My rule is to test unknown tubes with an inexpensive and disposable headphone before connecting the really good ones. All had tested good by the seller.