The Manleys will be voiced to use their own "E34Ls" tube supplied by Groove Tubes. It is a copy of the GE tube, made on original GE equipment bought by GT and setup at the JJ plant. It is a totally different tube from JJ's own EL34 and KT77.

I have tried a few other new production tubes in my Snappers, including cryoed SED, but the GTs are best by far. Buy them from Manley, they are just as cheap as GT but they screen and match better. They are $40/tube in bulk, so it would cost you 800 to retube the 250s. To me it would be worth it to hear what they are truly capable of. Also, the Snapper runs them hot, so 2000 hours is all I get, but maybe they will last longer on the Neo250. I've never heard the 250, but the snapper (100W) is an incredible amp. Powerful, extended, lightning fast, extremely dynamic, low output impedance and very low distortion. Also, Manley takes VERY good care of their customers.
I have a higher lust factor for the VTL than the big Manleys however. Heard at RMAF 07 with Theils. Very nice...