I've tried the Siemens 7DJ8. A friend told me they wouldn't sound as good as their 6v brethren due to the filament running cooler when in a 6v circuit but I got them anyway (they are usually significantly cheaper than the 6v options so trying them won't break the bank). I thought they still sounded pretty good. I can't say if they sounded less good than the 6v option as I didn't have the 6DJ8 from the same brand and time period to directly compare them to. Hope this helps!
