There is not a big difference between the Sovtek and the Electro-Harmonix. The Electro-Harmonix is ever so slightly smoother and sweeter than the Sovtek, but the difference was negligible in the Dodd Buffer Preamp that I had in my system for a little while. The Dodd is very, very transparent, and if the difference with the Dodd was so small, I can imagine that with the Modwright you would have a similar outcome.
Both the Sovtek and EH 6H30 are quite good, so my opinion is aligned with Teeshot's regarding the stock 6H30, which is almost certainly a Sovtek.
Having said that:
I agree with the general sentiment expressed on the thread that the Reflektor 6H30-DR tubes are a significant step up over the lesser Sovtek and EH tubes, but they are also at least 3x the price. It looks like PartsConnexion sells them for about $100 each. Do a search for 6H30 on the PartsConnexion site and you should see them listed. I think you can get matched pairs too for a bit more money.