Differences between different brands of VT-4C/211

I wish to buy a pair of NOS VT-4C/211, either GE, RCA or United Electronics for my Air Tight ATM-211 power amp, but I don't have the chance of listening to/comparing them.

What are the sonic differences between them? The information is hard to find as all of them are no longer in production and some of them (ie RCA and United) are very very rare.