The round plates I assume, right? I'm actually kind of hesitant to compare because I was listening to the greyglass when my EML tube was still new. I intend to pair that combo again here shortly and listen some more. The round plate tung sol is in a league of its own though. I had to recalibrate a bit to enjoy my other tubes again after taking them out.
The presentation is quite different between the two. The RCA exhibits a weight and thickness that contrasts sharply with the tung sol which comes off as sounding light and delicate. The fascinating part is how the TS does this without sounding slow or feeling like it is lacking in drive. There is a sense of finesse present that no other tube I've heard matches. The detail comes in layers where you only hear a flat plane of sound with other tubes. Tonally the greyglass RCA is more forward in the lower midrange, and has much less air from the upper midrange through the treble region. High notes don't project like they do on the TS. I think the TS midrange is more accurate, but the RCA suits some vocals well by lending added weight. Both have nice detail and speed. I think the RCA wins on PRAT by a little bit, but the TS has superior resolution and microdetail. More than that it is all in how things are presented where the TS really excels.
Truthfully its really hard to put into words what I want to say for comparison. They sound very different, but explaining that in words is hard for some reason. The tung sol is better, no question, but it is also way more expensive so it ought to be. I want to mention again my impressions of the greyglass are not fresh either so take this with a grain of salt. That, and I actually thought the greyglass RCA didn't have great synergy with my EML 274B. The detail got better, and everything gained control but they suffered tonally. The midrange loses much of the warmth this tube is known for, but again I think I need to break them in more and try again now that the EML has more hours on it. I've found that rectifier synergy really matters a lot in general. Some rectifiers I have which I wrote off as bad sounding have turned out to be pretty nice with different driver tubes paired up. It's all about identifying the characteristics in a tube you like and trying to pair those up with a tube whose characteristics will be complementary. Or you could just randomly try stuff and be surprised when a combo sounds good, like I've been doing for the most part.
I also have some t-plate tung sols which are more in the same league with the greyglass. These are basically tung sol mouse ear tubes only they don't actually have the mouse ears. If I had to pick one or the other I would take this tung-sol too, but again this really reminds me how much the rectifier can change things. Without the EML my answer would be the opposite. I really like how the greyglass RCA sounds on my 5R4GY (my previous regular use rectifier before the 274B) and thought the tung sol t-plates sounded detailed but dry and off sounding somehow. The EML literally fixed everything I didn't like about those tung sol tubes, and took something away from the greyglass in terms of portraying emotion in the music in exchange.
Something I would say in general is that I find myself just preferring the Tung Sol sound regardless of which particular tube it is. They have a sense of balance and neutrality that I've come to prefer more and more over time. Of course my EML is not losing its place in the rectifier slot on any long term basis, but I've been really surprised at how much I like the all Tung Sol combo I was running for the last few days (TS 5V4G and TS 6SN7GT T-Plate).
In the end all you can do is try things out and see what you prefer. What I like might not be what you like, and what headphones we're listening on is a huge x-factor too.
