Hey there.. Your cab should be wired in series/parallel as 8ohms (since you’re using 8ohm speakers.)
Honestly, the amp manual should tell you what’s compatible. You mentioned that it is a stereo amp / you have to run 2 cabinets. Does the manual mention impedance required for each amp? If it’s two separate amps, your only concern is to meet the demand of each amp. So if the only option is 8ohm, you’d be best to have 2 – 16ohm speakers, wired in series. You still have the option of running your 2 – 8ohm speakers in series, but you’d be running a 16ohm load.
I don’t know much at all about the amp, but if it’s powered by tubes, it’s probably going to be loud enough. That said, a lot of that depends on how you eq your tone. Mids will triumph at high volumes!