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.
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.