Tube amps and speaker ohms

Hi ...I like my solution to the same situation you have. I like the SET tone but I want to be able to use a wide selection of speaker. The answer for me was the TAD 60 power amp. It is very flexable in that it is rated at 30 watts in triod and 60 watts in UL mode using kt88 or 6550 powers tube. Using el34 , 6ca7 and the like has a little less. However the neat thing about this amp is that you can bias it into Class A and it sounds very much like an SET

Anytime you are investing in tube amplifier technology, that investment will be best served by a speaker that is at least 8 ohms or more.

Tube amps, with rare exception, have reduced performance into four ohms.

Given that you have satisfied that part of the issue, the next is efficiency. 40 watts with a speaker that is only 86 db is not going to be a lot of power unless you happen to listen nearfield, regardless of the type of amp.

SETs in particular have certain advantages of low level detail over conventional push-pull, but to tap that advantage you will need a speaker that is a good 10 db more efficient if your amp is 40 watts! Otherwise although it might work 'OK', it certainly will not be meeting its design goals.

40 watts into 89db is pushing your luck. Anything loud or dynamic may over-tax your amp and distort. Also, if you are new to tubes, 845's are not necessarily the best way to go. They are pricey and difficult to "roll" when compared to EL34's or KT-88's.

From what I know, lower impedance may run at slightly higher distortion levels. Many tube amps can handle 4 ohms or less but 8-16 ohm speakers are usually much more tube friendly.