From the JBL 104 setup manual
Connect your sound source to the master speaker using one of the following inputs:
- The AUX (3.5mm) input can be used to connect to personal devices such as cell phones, laptops,
and MP3 players.
- On the back panel, you have two input options. You can connect professional equipment with high (+4 dBu) outputs, such as audio interfaces and monitor stations, to the 1/4” (TRS) inputs.
- RCA Left and Right Inputs can be used to connect consumer equipment with low (-10 dBV) outputs, such as CD players or turntables.
There are three "reference" levels for audio. In general 'pro' audio runs at +4dB, 'semi-pro' runs at -10dB & 'consumer' runs at -20dB. Connecting the wrong ones together is never great, though rarely immediately damaging, so long as you watch your levels.
The expected levels for your three options above are -20, +4 & -10 (fortunately they expected these to have semi-pro use).
The +4 inputs on the monitors expect balanced line, TRS jacks.
Not all jacks are TRS, in fact in most semi-pro & pro applications only TS jacks are used. That includes everything from guitars to keyboards (with some exceptions).
The aux jack will almost certainly be expecting -20 from phones, laptops etc.
That leaves the RCA connectors.
From the Korg SV2 spec…
Audio Output L, R connector (XLR balanced), L/MONO jack, R/MONO jack (6.3 mm [1/4"] unbalanced)
So, that gives you two options for cabling to match levels.
2x XLR to 2x TRS balanced.
2x TS (standard guitar jack) to 2x RCA phono.
Jack to phono will be much cheaper if less rugged. For practise, durability may not be a factor.
As you've discovered, plugging TRS from a TR socket at -10dB to TRS expecting balanced +4dB is mismatching your levels by a factor of 14dB or so depending on how you measure it… that's why it's so quiet.
I'm not considering the differences in dBu & dBV standards for this, just to simplify. More info at biamp-Gain structure: input and output levels for the inquisitive.