Input impedance is 50 kohms. Page 2 below. The 33 ohms is probably for its electrical load.

.5 volt input for rated output.

Direct connection should work, don't see which type of input jacks it uses. As long as you can adapt the sound card out to the type jack at the input everything should be good. No need for anything else.

BTW, Welcome to ASR, just noticed you were a new member.

Could it be that the total impedance of all the amplifiers connected on the same audio line input signal (there might be up to 50 amplifiers) may go as low as 33 Ohms ?

How do we calculate the total input impedance of 50x amplifiers in parallel (if the input impedance is 50K). Is it like 50 x 50K resistors in parallel ?

I'm confused about this 33 Ohms, why do they sell a 600 Ohms to 33 Ohms converter

GaiTronics 370-301 600 Ohms to 33 Ohms Converter
On sheet 10 It says 33 Ohms max. audio Out is 1.5Vrms.

I took measurement on 3 audio lines (different number of amplifier and total cable length) with a Tenma Model 72-6948 audio impedance tester and got these readings: 14 Ohms, 20 Ohms and 23 Ohms.

If 1,5 Vrms is required on a 33 Ohms impedance audio line it needs 0.068 Watt of power, does a computer sound card produce enough power for this ?

IF not, is there some preamps that are having a mute input signal that could do the job ?

I was also thinking of using a small 2 watt / 8 Ohms amplifier like

Vicking PA-2A and add a resistor on the amplifier output to adjust the total audio line impedance to 8 Ohms (for example, 20 Ohms line + 15 Ohms resistor = 8.5 Ohms total) but I would get an audio signal maximum voltage of 4 V rms, this is too much voltage for the speakers amplifiers (should be somewhere between 0.5 and 1.5 VRMS from the documentation). Any tricks I could use to match it ? (600:150 audio transformer ?)

Thanks