I’m honestly not sure what stores you have there and the availability
You need something like this https://www.amazon.com/AmazonBasics-1-Male-2-Female-RCA-Y-Adapter/dp/B01D5H8RE0
For some reason I could find anyplace right now that sells them online with items in stock
Most electronics stores should have them
On the low side: Kicker 43DXA1252 Car Audio 2 Channel Stereo Speaker Amplifier 250W Amp DXA125.2 (Renewed) https://www.amazon.com/dp/B079VV8QQK/ref=cm_sw_r_cp_api_glt_i_AT1PA2GZX4H8B8FAHCZR
On the high side: KICKER CX400.1 400 Watt Class D Mono Amplifier for Car Audio Speakers, Black https://www.amazon.com/dp/B07Z9LBXNG/ref=cm_sw_r_cp_api_glt_i_KA8HT9E6AAC2BZ5HH7AB
The 400 watt mono is very powerful and clean. One of mine is still going after 7 years. It’s pounding a Kicker Solorbaric 10” sub that gotten in me pulled over twice for noise. You won’t need anything more.
How did you get the signal from the factory radio to the amplified subwoofer? The kicker products are pretty bullet proof. I’d check:
Signal cable (try another RCA)
Redo your ground to the amp/box
Check the signal converter from the factory radio to amp. Here is one Mark recommends that I’ve used twice from Audio Control: AudioControl LC2i 2 Channel Line Out Converter with AccuBASS and Subwoofer Control https://www.amazon.com/dp/B00IIL0LW0/ref=cm_sw_r_cp_api_glt_i_AAQE592YNJSAWHW131BV?_encoding=UTF8&psc=1
Install a ground loop isolator: InstallGear Ground Loop Isolator Amp Noise Filter https://www.amazon.com/dp/B077Y5DLBB/ref=cm_sw_r_cp_api_glt_i_DESNT1B4Q2S40FC9PB02
One of these should work. Good luck
ALITOVE 12V 15A 180W Power Supply Transformer Switch AC 110V / 220V to DC 12V 15amp Switching Power Adapter Converter LED Driver for LED Strip Light CCTV Camera Security System https://www.amazon.com/dp/B078RZ6C3N/ref=cm_sw_r_apan_glt_i_SGTVK6BP9E5QKRSS938M
Is that one of these - https://www.amazon.com/Quimat-Oscilloscope-BNC-Clip-Assembled-Finished/dp/B07QML4LJL
Anyway - onto the subject at hand. Keep in mind that I'm basing this on my applications engineering background so it may not be the case in this specific instance. One thing to consider is that mobile electronics are considered commercial electronics and use commercial grade internal components. Every component has a tolerance (rated in percentage, 5-10% is typical of commercial grade parts) and design engineers can compensate for it in some ways but you can't get around component tolerances. That being said, the 6.5V max rating is a typical max and so what I suspect may be happening is that the actual combination of component tolerances might result in a real world input limit of less than 6.5V; probably somewhere around 6.4xV. I would turn back the output voltage of your signal processor to somewhere around 6.175V (6.5V -5%) and check again.
It looks like that amp only has 2x25amp fuses on it so you don't need 1/0 to it and you definitely don't want them nearly touching either. I would recommend you terminate those 1/0 lines into distribution blocks like these then do a short run of 4 or 8 ga to the amps.
See this chart for amp capacity over distance. Also remember it does not matter if you're amp says 500w or 5000w it can never draw more than the fuses on the outside allow.