Really...My favorite power tube is reissue that's not widely popular

@soundermn 
A vintage 7581A should be closest to a 6L6GC, and then to KT66. It's not really "that" close to an EL34, but according to Prima Luna's manual the EL34 setting is your best choice available and should work fine though it will somewhat reduce the power output available from the amp. I wouldn't expect the power reduction to be TOO much; maybe say 30%?

@mulveling Thank you for the follow up!  I actually stopped playing in this forum, because I got tired of so many jack asses who have to pontificate and condescend in every darn thread.  I'm sure you know who I mean.  But, this one thread sent me an email because I'm subscribed, and I wanted to update on my progress.

To update, I did use the 7581A for a while, and I really liked them.  After I got new speakers, Revel F228Be, I did a bit of a shoot-out with KT150, EL34 and the 7581A.  The winner by a small margin was the stock Primaluna EL34, then 7581A and last KT150.  I sold the KT150s, and still have the 7581 octet, but would sell them if anyone is interested (Under 500 hours, fair price, PM me).  I don't see going back at this point.  And, to your point, I did run them on the EL switch setting.  They were very nice, the price is right, and I can see that with a different room or speaker it could go the other way.

So, what I ended up doing is I got better EL34.  I ordered TungSol EL34b, and they were definitely an improvement over the PL EL34.  Like the 7581 they are priced right, and the sound is clearly the best of all I have tested.  While the 7581 sounded better than the KT150, the EL34b was another step in the right direction.  I was surprised, because I thought the KT150 would "grip" the large speakers the best, but it didn't work out that way.

I don't expect much change in the system for a long time... I've reached the nirvana I wanted.  And, I'm intending to stick with what I have, rather than try to improve every month or two... and I'm just enjoying it all!

PL HP Integrated
Vintage JVC VL8 with AT ART9 cart
Manley Chinook
Revel F228Be
Pair REL T9/i subs
EL34b power tubes
Cifte Input tubes
PL rectifier tubes.

What improvement did you hear? Can you describe? I have 4 pairs of 7581As. Thanks.

@soundermn 
Awesome, glad you found out your favorite tube and personal nirvana, even if it was with the original EL34! Funny, I tried KT150 in my own amps (VAC 200iQ) earlier this year, and like your case they came in dead LAST of 3 sets - versus Tung-Sol KT120 and Gold Lion KT88 reissue, in my case. The bottom end was VERY powerful, dynamic, and punchy, but in the end it just swamped the music and sounded unbalanced. Sounds like my amps were really designed for KT88, but the KT120 are similar enough and actually eek into the #1 spot for my tastes. The KT150 just sound like a different species compared to those two, but I realize they might be amazing in amps designed specifically for them.

@noromance My experience was similar to @mulveling .  But, it's definitely speaker-dependent.  Before the new speakers, I had B&W 803N and then Revel F208.  For both of those, the KT150 was better than EL34 overall, although I'd say that the EL34 did have a sweeter midrange (or it seemed that way because the bass was less punchy).  KT150 retained a lot of the flavor of the EL34, but had more punch in the bass, well balanced across all frequencies, and IMO better control of the speakers.  When I moved up to the F228Be, the KT150 sounded good until I started to compare.  The 7581a are really nice (better than KT150 overall), and really close to the EL34, especially in midrange.  But, somehow the EL34 seemed to have cleaner, deeper bass, and clearer, crisper highs.  All of that was definitely improved further when I got the TungSol EL34b.  And, on top of tha, they have that liquid midrange wonderful sound that EL34 gives you.  It's just better all around.  

Again, it was a completely different outcome with different speakers.  So, you have to try for yourself.  If you still have EL34 and 7581, do a comparison and see which you like.  

Interestingly, @mulveling , I didn't like the KT120 nearly as much as the rest of these.  I found them to have a certain "glare" in the midrange, especially around vocals.  For example, Mark Knopfler's voice always sounded a little "fuzzy" around the edges.  With all the others, that effect is completely gone. But, again, it is probably speaker dependent.  I didn't have any KT120s to try with the F228Be speakers, so I can't validate.  If it works for you, then spin some vinyl (or shiny discs) and enjoy the music!