New tubes, possible matching difficulties

As a newcomer to tube integrated amps, some general advise please. I recently switched from tube pre(CJ Premier 17) and SS power amp, to a tube integrated. I planned for a tube power amp, but rapidly concluded, rightly or wrongly, that you get more bang for your bucks with a tube integrated. I bought a Viva Solista 845 SET operating in Class A and putting out 17 watts. I am very happy with the results, but as usual, am looking at possible tweaks. The unit uses 211's as rectifiers and standard 845's as output valves. Having poked around on the asylum SET forum, I have come across the Shuangung 845B, suppied by TNT.
One limitation or advantage perhaps of 845 valves, is that there are'nt a great range of expensive alternatives, as with the 300B, only ruinously expensive NOS units. The 845B seems to have a good report and are not that costly. However, I noted one report of a purchaser who could'nt get the valves to function in his, I believe De Haviland 845. I am not sure of the reason, perhaps differing plate voltage, I am not competent to speculate. Is there any way of judging how likely it is, that a tube swap would work in my amp? The superTNT site do not have a contact system to respond to queries, presumably the language difficulty.
In passing, any views on Tube Rings? If they do work, where would you start, Driver, Rectifier or Output tubes?
Thanks for looking and any words of wisdom you might have
Please specify what driver tube your Viva uses. I have done some experimentation with the Viva Aurora.
Sorry, I should have been more specific:
2, 845 output valves,
2, 211 Rectifier
2, stock 6SN7GT Drivers,
I live in the UK and Nick, the UK Viva importer I know quite well, has suggested that upgrading the drivers is an option. The Aurora uses 845 valves as well I believe, all be it on a grander scale, so I would very much appreciate your experience with the Aurora and tube or other upgrades.
Are you sure the driver is 6SN7 and not 6SL7 like the Aurora?

If 6SL7 there is a US Military tube that will make more difference than all the other tube rolling combined. Check and let me know.
I checked, the drivers are 6SN7EH, electro harmonix units. If you have any other ideas I would be grateful, but as you are the second person to suggest a driver upgrade, that is probably the place to start. With what, is another matter
The ultimate upgrade for the Viva Aurora is putting 1943 US Navy Sylvania 6SL7 tubes in place of the original Russian tubes.

I bought mine at Upscale Audio. Might check and see if they have any left.