Well, the ECC88s did not show up today, much to my disappointment. However, the 7308 did show up today and as promised I have some impressions to share.
As you can see in my sig, I've become pretty attached to the rocket logo 6N23P. It is a good tube, and it also has price going for it. I paid $17 for the rocket logo, whereas the 7308 cost me about $95 + shipping. The 7308 I bought was a platinum grade tube (added $10 to the $80 list price) & it was cryo treated (added another $5). With tax/shipping it came out to $115. $115 is obviously a lot more $$ than $17 so keep this in mind when you decide what tube is best for you.
Right away, even in the first second of starting a track, you can tell immediately that the 7308 is a MUCH better tube than the Russian tubes. It retains the wonderful warmth of the rocket logo tube, but adds to it a level of clarity and detail from the bass to the uppermost treble.
On the subject of detail, it's really stunning ... I can hear fingers brushing strings, the creak of chairs in the studio, it is so revealing in fact that lower quality rips that would have sounded OK with the rocket tube sound a little less good on this tube (note to self: re-encode those 160Kbps mp3s).
As far as actual sound, everything is there -- the bass is liquid yet refined, mids are smooth and full and the treble is beautiful and not tiresome at all.
Sound stage with this tube is as close to perfect as you can expect from a $200 amp & $400 set of cans, I feel like I am listening to something much more expensive.
So I think this one is a keeper, and it will be my primary tube going forward. Was it worth $100 more than the rocket tube? I'm going to say yes, but if you're on a budget you can skip this upgrade without missing too much (but you'll be missing something).
Hopefully the ECC88s come tomorrow so I can compare. I'm going to go lose myself in music now