My two favorite smaller to mid-size coils are the 5" DD and 7" Concentric. The 7" Concentric if my favorite all-purpose coil on that unit. These coils are made by Makro.
Headphones and audio report are another thing. The speaker produced audio of the Multi-Kruzer is very crisp and clear. However, due to the speaker impedance and design of the wireless headphones, they have a more-or-less 'muffled' sort of sound for me, too. The reason is that I have impaired hearing, actually deaf according to the military since 1982. I own and use three brands of corded headphones because some detector models I have don't work very well with one so I grab the set that works best with them.
On most of my detectors, the Killer B 'Hornet' headphones work quite well, such as with my Nokta FORS CoRe or FORS Relic and Makro Racer 2, or my White's XLT or any of my Tesoro's. But the Nokta Impact and Makro Multi-Kruzer and White's MX-7 still don't sound as loud or as clear so I use a different headphones such as my White's Pro-Stars. My 3rd set of headphones are the Detector Pro Uniprobe so I have three different types, all with the tank-style ear-cup, to choose from for best audio loudness and clarity.
Monte