New Sensor is the US importer, and they name-brand these tube lines for this market. Both lines are manufactured in Russia in the Reflektor factory (as are some other brands imported by New Sensor). I'm not sure this applies in every case, but for the types I've seen that are offered in both lines, the Sovtek and EH tubes are actually physically distinct from one another. But personally, when it comes to new-manufacture miniature 9-pin, 12-volt dual-triode varieties (such as 12AX7 and 12AT7) in specific, my preference has usually been for the Yugoslovian EI make. (Also over the SED, JJ and Chinese versions I've heard.) In my experience good examples of the EI can sound almost or equally as good for less money than some of the moderately-priced varieties of NOS tubes, although not as good as the better varieties of NOS tubes (which of course can cost many times their price). But despite that they're brand new and seem acceptably durable in use, you're faced with a high percentage of noisy or otherwise sub-par tubes straight from the production runs (not saying this is unique to EI), even after the manufacturer's and/or importer's testing and QC, so it's still a good idea to deal with a reseller who'll test, grade, match and guarantee their tubes' performance, especially for high-gain preamp/phono stages. It's also often worthwhile to try some inexpensive tube-damping polymer rings with these types.
