Hello
@acg
Where are you located? finding the best E280F is not as easy at searching for a particular brand. We do have a company internal rating system, but even at the lowest 1/10 rating, the tube sounds magnificent.
Reading above, i suspect you are a tube person(maybe understatement?) so what i would do , it something else.Here is a list of tubes you can search that work on your Kassandra.Which is better depends on the whole system,speakers etc and especially your ears.
List below is type, brand(usual ones but not all of them), recommended bias range(not necessarily hard limits ) and subjective range(depending on brand/batch)
E280F, Siemens, Mullard,Phillips, Tele -200/-280 1~10
E180F Mullard Phillips, -170/250 1~5
E810F Siemens, Phillips, Mullard -220/-330 1-8
6j11p Russian various -190/-300 1-8 !
D3A Siemens -180/-270 1-4
There are some more types, but these can keep you occupied for a lifetime
The E810F is now standard in the recent units going to US/Canada side. The E810F goes quite well with the Kass and the Ianus range.
The 6j11p you can get them in the dozens easily and easily last quite some years.
Do not underestimate the E280F's life time, i tell our users to not change it until it dies. This tube gets better as it ages.Do not change if it is working on. We had an amp playing 7.5 years (daily use) with the original tubes installed.Some dies earlier but we have a lot of units playing for many years with their original tubes.
Cheers
s