With that amp, I wouldn't use an open baffle speaker, like the Diesis Roma. No matter what the intentions are, OB are the most unsensitive type of speaker you can buy (unless it doesn't have bass extension). Can't fight physics.
Look for a speaker with an impedance higher than 8 ohms, and as flat as possible (and a true +90db sensitivity).
Something like this, on paper, would be as close to perfect as possible:
Casta Acoustics Model C:
Look at the impedance plot:
98db 1 watt with corner placement.
I haven't heard one, but on paper looks like a match made in heaven.
Look also at Haigner speakers, flat, high impedance and very high sensitivity.
It's very, very difficult to find an optimal speaker for a tube amp (8 ohms flat or minimum impedance), good luck with your search!