here is post #2078 p. 139 (happy to see that you agree with the reviewer):
Here is a review of the IBM 1680 (6BE6) tube made by Sylvania: (please read until end or skip to summary)
EF95 1/7 Strap setting (used for burn in): The first impression was the signature heptode sound of wide expansive sound stage, clarity and abundant detail. Then some shortcomings became apparent: soft flabby bass, cool mid range, lack of dynamics, and sibilant treble.
These deficiencies disappeared slowly, and after significant burn in they almost all disappeared to a certain extent. After being burnt in for 50-60 hours the tubes had improved markedly. The bass is acceptable but lacks slam and attack. The mid range became warmer. The highs are still sibilant and sharp. Good bye (and good riddance) EF95 1/7 setting.
EF95 NS (no strap): This setting yields better sound than the one previously described. Firmer, more distinct bass, mid range regular warm, highs still sharp but no sibilance, good detail. Yep, much better, but no wow factor. A certain lack of micro dynamics, rhythm and pacing. A good simulation of my solid state receiver with better bass. Bye Bye EF95 NS.
EF95 2/7 strap: Wow! Multi layered 3D heptode magic! Bold, powerful bass, full of details and micro dynamics, great highs. Involuntarily toe tapping - a 4 on a scale of 5.
Cons: Bass does not go down to the deepest levels, and there is a minor mid range coolness and hollowness.
Very good presentation overall with tons of detail and musicality. EF95 2/7 - you are welcome any time.
EF92 setting (using EF95 6/7 strap setting): Treble excellent with great detail and power but some sibilance; mid range good, bass is a bit boomy. Overall quite good, grows on you. Perhaps this setting would match certain types of equipment. EF92 - nice to see you, bye.
SUMMARY OF THE IBM 1680 TUBE:
As mentioned before, the differences between heptodes are much more subtle than between 6AK5 tubes, and thus harder to describe.
Using the EF95 2/7 strap setting this tube is very good performer, albeit not on the level of the very best heptodes.
The EF92 setting may be a good choice if you like a very detailed treble presentation.
EF95 NS is OK.
EF95 1/7 is not recommended.
Price wise, the 1680 is a winner with a cost of less than $8 for a pair shipped.
As they say in a prominent consumer magazine: Recommended, Best Buy.
Edit 7/27/13: Listening to the EF92 simulated setting now. Don't know if the tubes stabilized or my perception changed, but these tubes sound excellent now on this setting. Bass is firm but not in your face, almost no sibilance. Detail, detail. Simply beautiful.