When an amp puts out 90 watts....

Fin,
I think that is indicative of sn amp with higher output impedance, usually but not always an attribute of tube amps. An amp like that will probably perform best into higher impedance speakers, ie 16ohms, which is not common. Its a special breed that needs a special mate. My recollection is that pbn is probablyof the more typical ilk, impedance of 8 ohms or less. Probably not the ideal pairing, but workable.

I have seen some pnm impedance measures in stereophile that show wide impedance flucuations from 4 to 16 ohms at various frequencies. Far frm the constant high impedance that an amp like that would probably do best with.

The OP's Butler Audio amp is not a full tube design. I believe it is solid state with a tube input stage. Therefore it will not necessarily need a speaker with a flat impedance.

Finsup, It sounds like you are not happy with the sound at present. There are allot of reasons why that could be. If you could explain more about what you're not liking and maybe some details about your room dimensions maybe someone can give you some suggestions.

I have seen some pnm impedance measures in stereophile that show wide impedance flucuations from 4 to 16 ohms at various frequencies. Far frm the constant high impedance that an amp like that would probably do best with.

That depends on the amp. If it is OK with 4 ohms and has a 10db of feedback or more, its probably not an issue. Also, Sometimes you can get by without the feedback if the higher impedances also occur at higher frequencies. A ZOBEL network might just take care of the extra output.

I have heard from an amp designer that he has abandoned the Zobel because the Zobel network can knock down power output into the higher frequencies and diminish extension. If the speaker efficency is near 96 plus, less of an issue. Have you experienced this? Jallen