Voltage determines loudness level, not current. You need more voltage gain in your electronics, which is exactly what you're seeking with the +6dB gadget. Moar voltage!
https://www.soundlabspeakers.com/wp-content/uploads/2019/07/A3-Gordon-Holt-Sept-1986.pdfThey're only 76dB/w efficient, but it's only rated for 100 watts input power. Unfortunately, that means they can't play loud as you want no matter how you adjust the electronics. The theoretical maximum SPL at 100W is around 95dB. So uncompressed popular music (classic rock) will need to play with volume set around 80-85dB to accommodate peaks at speaker max power rating. Classical symphonies will sound mostly like 70dB to allow headroom for the finale peak. And because it's a panel speaker, as you approach full rated power the dynamic peaks will get more compressed, so you're kinda swimming backwards upstream.
If you like loud dynamic music then you need different speakers with a lot more SPL headroom. Fortunately, speakers have evolved significantly since then, so you have a lot of choices.
What's a good 6dB gadget? imo, Most of the inexpensive ones sound pretty bad, like burson, etc.
If you can DIY, or know someone who can, then make a simple LM4562 preamp with 10dB voltage gain. It could be powered with a 9V or 12V battery. One opamp plus a couple resistors, battery, i/o jacks and a box. <$100. There might be enough control voltage inside the Tortuga to power the opamp and wire it right inside.
Similar idea but the electronic design is already done, but it's more complex than you need with BAL I/O
https://neurochrome.com/products/universal-bufferI don't know of any inexpensive finished goods preamp with 10dB gain that's good enough for high end audio.
