The 6p3s-e has similar properties to the 6L6GC, which has long been a known acceptable sub for EL-34's in the ST70. Based on reports of this I purchased a quad from a vendor on Ebay located in Russia. Cost was $27 for the tubes and $10 to ship to Pennsylvania. Shipping took about 3 weeks from the time of my payment. Build quality seemed quite good and it should be as these tubes were designed for military application. All four tubes were in original boxes with spec sheets (in Russian, of course). The tubes had obviously not been tested by the seller.
The 6p3s-e's biased up very nicely in my amp and seemed to draw current very evenly. A good match even though they weren't tested. Initial listening impression was somewhat lacking in bass response although mid and highs sounded quite nice. After roughly 100 hours of burn in time, the tubes have opened up nicely and bass response has greatly improved. Overall, my impression of the Russian 6p3s-e is good, especially considering their price. I've decided to leave them in that amp until further notice.
I will note that I have only 1 year experience working on tube amplification circuits and that I am in no circumstances an expert on their operation. Therefore if you decide to try these tubes in your amp and they cause it explode violently, I deny any responsibility, but I think you're gonna like em!