I think you are doing the right thing as is. Each person hears differently so the best way to find the tubes that suite you are to try a few and find out from personal experience. I can give you general impressions, as can others, but we each have our own differing opinions. You've chosen two good tubes to start with. The Sylvania is going to have more energy on the top end...... more "sparkle" in the treble and probably a leaner sounding bass. The RCAs have a slightly thicker and more lush sound, especially in the midrange. It's all about finding the "flavor" you prefer so to speak. Give them both time to break in (10 hours minimum before making comparisons between the two) and let yourself get used to the sound of each one. What headphones you are using matters a LOT here too. I avoid Sylvania tubes personally, not because they are bad, but because I use AKG headphones and the combination results in too much treble energy. Many people like the Sylvania with Sennheiser and Denon cans, OTOH.
If you can afford it, go for the Sophia or the EML rectifier. It improves every aspect of the sound, like taking a general step up in performance all around. Regardless of which drive tubes you use upgrading to a better rectifier has benefits. I personally have found that the rectifier impacts soundstage, PRAT (pacing, rhythm and timing), and bass speed/impact a lot. Rectifiers also seem to have the biggest influence on resolution and inner detail. It doesn't seem to swing the tonal balance by a large amount. The power/drive tubes influence tone a lot and can add or subtract a considerable amount of bass/mid/treble depending upon the type and brand of tube. They also can affect PRAT and bass, but not to as large an extent as the rectifier. They also affect soundstage less, though they still do somewhat and I've found the "character" of the sounstage changes a lot with different power tubes. Like spatial perception of width vs height for example or how much perceivable space each instrument or note is given. Attack and decay is heavily influenced by power tubes too, more so than the rectifier IMO. Keep in mind these are only MY personal opinions. You may hear something else entirely.
Lastly, I suggest trying out a 6FD7 tube as well. Whichever of your 6EW7s you like better, RCA or Sylvania, give that same brand a try in the 6FD7 so you can see the differences that tube makes. I really like the 6FD7 sound myself, so long as you get a good one. The older the tube the better, they tend to be more refined and have a more even tonal balance.
I wouldn't worry a lot at this point about some of the jargon. Mention of silver on top of your RCA tube, for example, means the "getter" is on the top of the tube. This is part of how a tube is made, and the reason buyers pay attention to it is because it can identify the vintage of the tube. Some designs and vintages perform better than others. This is why you hear things like silver top or clear top mentioned in a tube description, and you'll pick up on what this stuff means over time as you read more. One important distinction is "fat bottle" vs "thin" or "tall" bottle. A fat bottle version of the same type of tube will sound different. I like fat bottles over thin bottles where it is applicable personally. Some tubes were only made in one type, the 6DE7 for example is only thin bottle. The 6EW7 and 6FD7 were made in both.
Just enjoy the music and have fun with it most importantly.
