The NDS has a rated output of 2.1V, meaning that is the maximum (the actual output will depend on the recording level and of course the music).
The 300 has a gain of 29 dB. 6dB is a doubling of voltage, so every 6dB doubles the 2.1V. 29 is nearly 5 doublings of 2.1V , so somewhere round about 60V.
Sopra 1 has a rated sensitivity of 89dB/2.83V at 1m (89dB/W into 8Ω). Every doubling of voltage gives an extra 6dB output, so 60V = ~4.3 doublings = ~4.3x6 +89 ~116dB at 1m
A second speaker playing precisely the same signal in phase adds 3dB. Doubling the distance from the speakers subtracts 6dB. So at 2m from the speakers, approx 116+3-6 =113dB. At 4m from the speakers it is 107dB.
The 300 has a transient capacity of 500VA. With 8Ω speakers 60V is nominally 450VA, so within its capability (though its continuous signal max is 90W, which is a fifth of this), however I am unclear as to how long the Supra 1 can sustain this sort of power input.
So, NDS feeding your 300 direct with no preamp into Supra 1s, listening at about 3m distance is likely to yield a sound level of up to about 110 dB on peaks. That is is very loud, but music with peaks up to that level could sound realistic with some programme material - though likely to be damaging to hearing with prolonged listening at that level, depending on what is the average level. (However, something with peaks at a very high levelcompared to the rest of the music - e.g. the 24bit Telarc recording of Tchaikovski’s 1812 overture - limit the maximum to 110dB may make the rest of the piece sound quiet.)
N.B. Many approximations in the above, but very much in that ball park.
A preamp (other than a passive one) on max is likely to be louder as the preamp adds its own gain, most being above unity.